The journey through this book

This chapter is a road map of the whole year. It runs from tiny microbes we cannot see all the way to challenges as big as the planet.

Tiny life. One drop of water hides a world of tiny living things. Some are helpers: they help us digest food and make medicines. Others are harmful and cause infections. That leads to health: what the body needs, and how good food, exercise, medicines and vaccines help us fight infections.

Electricity. The heating effect of electric current keeps us warm. Its magnetic effect makes motors run and machines work.

Forces. Electricity's effects depend on basic forces, so next come forces themselves: pushes and pulls that make things speed up, slow down or change direction. They explain why a ball thrown up falls back, and why a car stops when the brakes go on.

Pressure and weather. Pressure is how a force is spread over an area. Air moves because of pressure too. A small difference in pressure gives a gentle breeze; a big difference gives strong winds, and sometimes cyclones, which affect our lives, our farms and our safety.

Particles. Everything is made of tiny particles. In solids they can hardly move; in gases they move about freely. Materials can be sorted into:

• elements: pure substances;
• compounds: two or more elements bonded together;
• mixtures: things mixed that can be separated again physically.

That explains solutions, such as how the sugar in your chai disappears and makes it sweet.

Light. Light bounces off flat and curved mirrors and bends as it passes through lenses. That explains the funny image in a shiny steel spoon, and how glasses help people see clearly.

The Moon and calendars. Rough surfaces reflect light too, and so does the Moon. Depending on where the Earth, Moon and Sun are, a slightly different part of the Moon is lit each night: its phases. Watching that cycle let people make the first calendars, together with sunrises and sunsets.

Life and the Earth. Every living thing, from the tiniest insect to the biggest whale, depends on air, water, sunlight and other living things: together they form ecosystems. The last chapter asks what makes the Earth just right for life (the right distance from the Sun for liquid water, and air that gives us oxygen and blocks harmful ultraviolet rays), and how human activity is changing its climate. We cause the problem, and we can use science to help solve it.