The Nauka Za Mlade website was created in collaboration with the EDUKOS Citizens’ Association, the First Kragujevac Grammar School and teacher Miloš Milosavljević, with the aim of popularizing and promoting science, technology and critical thinking.
NZM / Remember this in 30 seconds
Remember this in 30 seconds
Short explanations of questions about nature, the body and space. Every answer includes a source and enough context to keep a striking fact accurate.
01
Why does association between two things not prove causation?
means that the values of two phenomena vary together, but it does not by itself explain why. One may influence the other, influence may run in the opposite direction, or both may depend on a third phenomenon. Warmer weather, for example, can simultaneously increase ice-cream sales and the number of people at swimming areas. It does not follow that ice cream causes people to go swimming. Causal conclusions require additional evidence and research that separates the possible explanations.
Why can we not keep all information in working memory?
lets us briefly retain and process information needed for the current task. Its capacity is limited but is not described by one number that applies equally in every situation. In controlled conditions it is often estimated at a few meaningful chunks, with results depending on the task and the grouping of information. Existing knowledge helps us combine several separate elements into one chunk. Writing down an intermediate calculation can therefore free capacity for the next step instead of keeping everything in our heads.
Caffeine neither creates energy nor replaces sleep but changes how we experience sleepiness. Among other effects, it blocks receptors for , a molecule involved in regulating sleep pressure. We can consequently feel more alert for a while even though our need for rest has not disappeared. The strength and duration of the effect vary between people and depend on the amount and timing of consumption. Alertness after coffee therefore does not prove the body has received what sleep would provide.
Why does CO₂ warm the atmosphere despite being much less abundant than nitrogen?
The greenhouse effect depends on how a gas interacts with radiation as well as how much of it is present. Carbon dioxide molecules can absorb and emit in particular wavelength ranges. Earth’s surface loses energy through the emission of . Nitrogen and oxygen, which dominate the atmosphere, participate much less strongly in that process within the relevant ranges. Changing the amount of CO₂ therefore changes the energy balance even though it forms only a small fraction of the air.
Sunlight as it enters droplets, reflects internally and again as it leaves. Different colours change direction by different angles, producing a coloured arc. The geometry of a primary rainbow corresponds to a circle around the point opposite the Sun in the sky. The horizon usually hides its lower part from the ground, whereas a sufficiently elevated viewpoint can sometimes reveal the full circle. A rainbow has no single fixed location seen identically by everyone but depends on each observer’s position.
A visible cloud consists of tiny water droplets or ice crystals rather than invisible water vapour itself. The total water mass in a large cloud can reach hundreds of thousands of kilograms. That water is nevertheless spread through an enormous volume in very small particles. Tiny droplets fall slowly, and rising air currents can keep them within a cloud. When droplets grow and conditions change, they can fall as rain, so a cloud’s floating does not imply an absence of gravity.
A desert is defined chiefly by low precipitation rather than heat or sand dunes. Large areas of Antarctica receive very little precipitation and consequently form a polar desert. Its enormous ice sheet arose through the long accumulation of snow that melts only with difficulty in a cold environment. Ice thickness therefore does not tell us how much precipitation falls in a single year. Coastal regions are wetter than the interior, so the whole continent is not equally dry.
Water passing over rocks and through soil dissolves small amounts of mineral substances. Rivers carry these dissolved to the sea, while some material also comes from processes on the seabed. During evaporation, water enters the atmosphere but dissolved salts remain. Rivers also contain dissolved salts, usually at much lower concentrations. Ocean salinity also depends on processes that remove , so imagining the sea as a container that only gains salt forever is insufficient.
Satellite navigation calculates position using very precise radio-signal travel times. Clocks on satellites do not tick at exactly the same rate as corresponding clocks on Earth. The satellites’ speed slows them in one respect, while weaker gravity at their altitude speeds them in another. For a typical orbit, the combined effects amount to approximately 38 microseconds per day relative to a ground clock. Even that tiny difference must be accounted for because timing errors become position errors.
Polaris is famous for its position near the north celestial pole, not for being the brightest star. Because of that position, other stars appear to circle around it during the night. Its height above the horizon roughly indicates an observer’s latitude in the northern hemisphere. Earth’s axis changes direction very slowly, so Polaris is not forever the only star that can serve as a pole star. For orientation, its location matters more than its brilliance.
Bluish light can appear around the Sun at sunset on Mars. Dust particles play an important role in its thin atmosphere, distributing different colours of light differently. Near the direction of the Sun, blue light becomes more prominent than the redder light scattered across other parts of the sky. The effect depends on the amount and properties of the dust, so sunset images do not all look identical. Rules for Earth’s sky colours should therefore not be automatically transferred to another planet.
Light takes approximately eight minutes and twenty seconds to travel from the Sun’s visible surface to Earth. We obtain that time by dividing the average distance by the speed of light. We therefore see the Sun as it was several minutes earlier. This is different from the much longer transfer of energy from the Sun’s core to its surface. Looking into distant space also means looking into the past, because no light signal arrives instantly.
Why is summer not caused by moving closer to the Sun?
The main cause of the seasons is Earth’s axial tilt rather than changes in its distance from the Sun. When the northern hemisphere tilts towards the Sun, days are longer and sunlight arrives at a more favourable angle for heating. The southern hemisphere then has winter because its lighting conditions are reversed. If distance were the main cause, both hemispheres would have summer simultaneously. Earth is actually closest to the Sun in early January, during the northern hemisphere’s winter.
The inner ear contains organs that register head movement and its position relative to gravity. The semicircular canals respond to rotation, while the detect linear acceleration and tilt. Movement of fluid and tiny structures changes the activity of sensory cells that send signals to the brain. The brain combines these with information from vision and receptors in muscles and joints. Balance is therefore the coordination of several information sources rather than the work of a single organ.
After encountering a particular , the immune system can retain memory cells. Vaccination uses this ability by presenting an , or instructions for producing it, without the risks of the full disease itself. A later encounter with the pathogen can consequently trigger a faster and more effective response. Protection depends on the vaccine, the pathogen and the individual, so it is not always complete or equally long-lasting. Additional doses help strengthen or maintain the response for some vaccines.
Bone is living tissue containing cells, blood vessels and ongoing renewal processes. remove portions of old bone material, while make new material. Their coordinated activity allows bones to adapt to loads and maintain their structure. In children and adolescents, formation can exceed breakdown because the skeleton is growing. Bone is therefore a dynamic structure rather than an unchanging piece of mineral.
A botanical berry is defined by how the fruit develops from a flower, not by its size or taste. A banana qualifies as a berry under this definition. A strawberry’s red edible part consists mainly of an enlarged receptacle, while its small surface “seeds” are actually individual fruits. It is therefore an aggregate accessory fruit rather than a true botanical berry. Culinary names and botanical classifications serve different purposes and need not coincide.
Are fungi more closely related to animals than to plants?
Fungi form a separate kingdom, although everyday speech often associates them with plants. Comparisons of molecular data show that fungi and animals share a more recent common ancestor than either shares with plants. This does not mean fungi are animals or that a modern animal descended from a mushroom. Fungi do not and obtain nutrients by breaking down and absorbing material from their surroundings. Evolutionary relatedness concerns ancestry, not merely similarities in appearance or way of life.
Plant cells carry out both day and night. This process breaks down organic substances and releases energy for their activities. is a different process, using light energy to build organic substances. With enough light, can outweigh respiration in overall gas exchange, so a plant releases oxygen on balance. stops in darkness but respiration continues, so it is incorrect to say plants only start respiring at night.
How can our eyes have a blind spot we do not notice?
Where the optic nerve leaves the retina, there are no to detect light. Each eye consequently has a small area of its visual field from which it receives no direct visual signal. We usually do not notice it because the two eyes’ fields complement each other and the brain uses surrounding information. With one eye closed and a correctly positioned dot on paper, we can demonstrate its temporary disappearance. This is a normal anatomical blind spot, not evidence that the eye has stopped working.
Most human is in the cell nucleus, but also contain a small amount. These organelles help convert energy from nutrients into a form usable by the cell. Their own provides instructions for some of their components, while nuclear genes encode many others. Human is normally inherited from the mother. It makes up only a small portion of our genetic material and does not replace the genes we receive from both parents.
Antibiotics act on particular bacterial structures or processes, such as the construction of cell walls. Viruses lack the same cellular machinery and reproduce using their host’s cells. An antibiotic therefore does not eliminate the virus responsible for a cold or influenza. A bacterial infection can sometimes develop alongside a viral illness, but that is a separate question for a doctor to assess. Unnecessary antibiotic use increases favouring resistant bacteria.
Human blood is red even when it carries less oxygen, although it is then darker. Its colour comes from , a protein in red blood cells. Veins can look blue because of how different wavelengths of light pass through skin, scatter and are absorbed. The vessel’s depth and the surrounding skin colour also affect the appearance. Blue in circulation diagrams is therefore a convenient distinguishing label rather than blood’s actual colour.
Chromium in stainless steel enables a very thin protective surface layer to form. This impedes further reactions between the metal and its surroundings and can renew itself under suitable conditions. Chlorides, some acids and unfavourable conditions can nevertheless damage that protection locally. Different types of stainless steel are therefore used for different purposes. Stainless means high resistance to corrosion rather than absolute indestructibility.
How can diamond and graphite contain the same element?
Both diamond and graphite consist of carbon, but their atoms are connected differently. In diamond each atom connects to four neighbours in a strong three-dimensional network. In graphite the atoms form layers, with much weaker bonds between them. Those layers can slide, allowing graphite to leave marks on paper, whereas diamond is extremely hard. Atomic arrangement can therefore change a material’s properties as dramatically as changing its chemical composition.
Microwaves are whose individual lack enough energy to atoms as X-rays do. In food they stimulate the movement of polar molecules and , converting energy into heat. Food therefore does not become radioactive or retain microwave radiation after the oven is switched off. Heating can nevertheless be uneven, so a meal that feels warm outside need not be equally warm at its centre. The claim that microwaves always cook from the inside out is also incorrect.
Dissolved salt lowers the temperature at which water freezes. If a thin layer of liquid water exists on the ice, salt dissolves in it and can encourage further melting. Ice and salty water can consequently coexist below 0 °C. The effect depends on the amount of salt and the temperature, so table salt is not equally effective in every frost. Spreading salt does not warm the road: it changes the equilibrium conditions between ice and solution.
A soap molecule has a part that interacts well with water and a part that associates with grease. In water, these molecules can form groups called , helping keep grease dispersed. Rubbing separates dirt from a surface, and rinsing carries away the resulting mixture. Soap therefore does not turn all grease into water but makes it easier to remove with water. Many detergents use a similar principle, although their compositions differ.
Sweat cools the body most effectively when it evaporates from the skin. Changing water into gas requires energy, which the water also takes from the skin, cooling it. Sweat that simply drips off does not provide the same cooling as an equal amount that evaporates. In very humid air, evaporation becomes harder and the body loses heat less easily. The same thermometer reading therefore does not necessarily mean the same sensation of heat.
Metal and wood kept in the same room for a long time can have the same temperature. Metal nevertheless draws heat away from our warmer skin faster, making it feel colder. Wood conducts heat less effectively, so the point of contact cools more slowly. Touch therefore registers the rate of heat transfer as well as an object’s temperature. That is why the feeling in your fingers is no reliable substitute for a thermometer.
Sound is a mechanical wave: it needs matter whose particles carry pressure changes. In the near-vacuum of space, too few particles exist to carry ordinary sound to our ears. An explosion can still emit light, heat and particles that travel through a vacuum. Sound could pass through gas inside a spacecraft or through its solid structure. A loud cinematic blast in open space is therefore an artistic effect.
Air in a lightning channel can briefly reach about 30,000 °C. That exceeds the temperature of the Sun’s visible surface, around 5,500 °C. It does not mean a lightning bolt contains more energy than the Sun: we are comparing a local temperature in a narrow channel with a huge star. Rapidly heated air expands and makes the shock wave we hear as thunder. The peak temperature is very brief.
As water freezes, hydrogen bonds help form an open crystal structure. The same mass therefore occupies more volume as ice than as liquid water. Lower density lets ice float, with part of it below the surface. An ice cover can slow heat loss from a lake, helping organisms beneath it. This does not mean every substance expands when it freezes; many contract.
Why does water boil below 100 °C in the mountains?
Water boils when its vapour pressure matches the pressure outside it. Atmospheric pressure is lower at higher altitude, so boiling starts at a lower temperature. Food in boiling water can therefore cook more slowly on a mountain even though bubbles appear sooner. The precise temperature depends on local pressure, not altitude alone. The familiar 100 °C figure applies approximately at standard sea-level pressure.
Fossils show that early relatives of modern sharks lived hundreds of millions of years ago. The oldest evidence for shark-like groups predates the first forests of true trees. This does not mean any living shark species has remained unchanged for all that time: sharks evolved too. The comparison concerns evolutionary lineages and the appearance of trees, not the age of an individual animal. Exact figures vary with the definitions of a “true shark” and a “tree”.
Bees can detect ultraviolet light, beyond the range of human vision. Some flowers have UV patterns that guide pollinators towards nectar. Petals that look uniform to us can show a striking pattern to a bee. Not every flower has these guides, and bee vision is not simply human colour vision plus a secret extra colour. Their sensory world is organised differently.
Two octopus hearts pump blood through the gills, where it takes up oxygen from water. A third heart sends oxygenated blood through the rest of the body. The blood contains copper-based haemocyanin, giving it a bluish appearance when carrying oxygen. This arrangement helps cephalopods supply their tissues in the sea. It is one circulation system with pumps doing different jobs, not three separate circulations.
Laser measurements show the Moon’s average distance from Earth growing by about 3.8 centimetres each year. The Moon raises tides, while Earth’s rotation carries the tidal bulges slightly ahead of it. Their gravity transfers angular momentum to the Moon: Earth’s spin slows and the Moon moves into a higher orbit. This does not mean the Moon will soon escape; the process is very slow. Its rate need not have been constant throughout geological history.
Venus takes about 243 Earth days to rotate once relative to the stars, but about 225 days to orbit the Sun. In that sense its sidereal day is longer than its year. Yet the time from one sunrise to the next is about 117 Earth days because Venus rotates in the opposite direction from most planets. “Day” can mean either a rotation against the stars or a cycle of sunrises. The distinction prevents a catchy fact from becoming misleading.
When will the Atlantic be larger than the Pacific?
The Atlantic widens because new ocean crust forms along the Mid-Atlantic Ridge as plates separate. Around many Pacific margins, old crust sinks into Earth, so the Pacific basin is shrinking overall. The Pacific is still much larger, and there is no reliable calendar date when their areas might match. Plate speeds and boundaries change over millions of years; a new subduction zone could reverse today’s trend. Geologists can explain the process much more confidently than they can name a year.
Which muscle is largest, strongest and most enduring?
The answer depends on what we measure. Gluteus maximus is the largest muscle by mass and helps us rise, walk and run. The chewing muscle called the masseter can exert great force relative to its size, but that does not make it strongest by every measure. Cardiac muscle works continually throughout life and is often called the most enduring. Any ranking needs to specify mass, force, work or duration first.
Sunlight contains all the colours of the visible spectrum. Air molecules scatter shorter wavelengths, including blue, more strongly. Blue light therefore reaches our eyes from many parts of the sky rather than only from the direction of the Sun. At sunset light crosses more air, leaving more reddish light on the direct path to us.
