Why is the sky blue? Everyday science on seasons, floating ice, salty seas and lightning

Some of the best science questions are the ones children ask and adults quietly cannot answer. Here are short, accurate explanations of five everyday mysteries.

Sunlight looks white but is a mixture of all colours. As it passes through the atmosphere it bumps into gas molecules, mostly nitrogen and oxygen, which are much smaller than the wavelength of light. Molecules like that scatter shorter wavelengths far more strongly than longer ones. This is called Rayleigh scattering. Blue and violet light are scattered most, so blue light arrives at your eyes from every direction in the sky. Why not violet? There is less violet in sunlight to start with, some is absorbed high in the atmosphere, and our eyes are more sensitive to blue. At sunrise and sunset, light travels through much more air to reach you. Most of the blue is scattered away before it arrives, leaving the oranges and reds.

Not because the Earth gets closer to the Sun. In fact, Earth is closest to the Sun in early January, during the northern winter. The real reason is that Earth's axis is tilted by about 23.5 degrees. As Earth orbits, the tilt stays pointing the same way in space, so for part of the year the Northern Hemisphere leans towards the Sun and for part it leans away.

This is why the seasons are reversed between the hemispheres: July is summer in Seoul and winter in Sydney.

Almost every substance is denser as a solid than as a liquid, so solid pieces sink. Water is an unusual exception. Water molecules attract each other through hydrogen bonds. In liquid water these bonds keep breaking and re-forming, letting molecules pack closely. When water freezes, the bonds lock the molecules into an open, hexagonal crystal with a lot of empty space. Ice is therefore about 9% less dense than liquid water, so it floats. This matters enormously for life. Lakes freeze from the top down, and the layer of ice insulates the water beneath, so fish survive the winter.

Rain is very slightly acidic, because it dissolves a little carbon dioxide from the air. Over millions of years it has slowly weathered rocks on land, dissolving minerals and carrying them down rivers into the sea. Volcanic vents on the sea floor add more. When seawater evaporates, the water leaves but the dissolved salts stay behind. Over enormous spans of time salts have built up in the oceans, mostly sodium and chloride, the same ingredients as table salt. River water also carries dissolved minerals, but in amounts far too small to taste, because it is constantly being refreshed by rain.

Inside a thundercloud, strong updraughts carry water droplets up while ice crystals and soft hail (graupel) fall. When they collide, electric charge is separated: generally, lighter positively charged ice ends up near the top of the cloud and heavier negatively charged particles gather lower down. When the difference in charge becomes great enough, the air can no longer insulate it and a giant spark jumps, within the cloud, between clouds, or to the ground. The channel of air heats in a fraction of a second to temperatures several times hotter than the surface of the Sun, and the air expands explosively. That expansion is the thunder. You see the flash first because light travels far faster than sound; counting the seconds until the thunder gives a rough sense of distance, about three seconds per kilometre. What everyday question have you always wondered about? Ask it in the comments and let the community have a go at answering.