Cycles on Earth

Think about the seasons: spring, summer, autumn, winter — and then spring again. The steps repeat over and over. That is a cycle.

A cycle has two important features:

• The steps happen in the same order each time.
• It repeats — when the last step finishes, the first one begins again.

Because a cycle goes round and round, there is no real start or end. You can join in at any point.

Cycles matter a lot in science. The Earth does not get fresh deliveries of water or nutrients from space. Instead, it reuses the same materials again and again. A cycle is nature's way of recycling.

The most important cycle you will study now is the water cycle — the journey water takes between the sea, the sky and the land. Later you will meet other cycles too, all built on the same idea: materials being used, returned and used again.

The water cycle is the never-ending journey of water around our planet. The same water is used over and over.

The whole cycle is powered by the Sun. The Sun's heat is the energy source that lifts water into the air and keeps the cycle moving.

Water changes state as it travels. It is sometimes a liquid (in the sea or rain), sometimes a gas (water vapour in the air) and sometimes a solid (ice or snow). The water itself is never lost — it just moves and changes form.

Let's walk through each stage of the water cycle in order.

Evaporation. The Sun heats water in the sea, in lakes and on land. The heat turns liquid water into an invisible gas called water vapour. The vapour rises up into the air. (Water given off by plants, called transpiration, also adds vapour to the air.)

Condensation. As water vapour rises, it cools down. Cool air cannot hold as much vapour, so the vapour turns back into tiny droplets of liquid water. Millions of these tiny droplets together make a cloud.

Precipitation. Inside a cloud the droplets join together and grow. When they become too heavy to float, they fall. This falling water is precipitation — it can be rain, snow, sleet or hail, depending on how cold it is.

Collection. The water that falls gathers on the ground. It flows into rivers, soaks into the soil, or fills lakes. Rivers carry it back to the sea — where the cycle begins again.

A useful order to remember: evaporation, condensation, precipitation, collection.

Every living thing needs water. Plants need it to grow, animals need it to drink, and your own body is mostly water. So where does fresh water come from?

The answer is the water cycle.

• Most of Earth's water is salty seawater, which living things on land cannot use.
• When seawater evaporates, the salt is left behind. The water vapour that rises is fresh (salt-free).
• That fresh water later falls on the land as rain or snow.
• This rain fills rivers and lakes, soaks into the soil for plant roots, and refills the underground water that wells and springs use.

So the water cycle acts like a giant natural water-cleaning and delivery system. It takes salty sea water, removes the salt, and delivers clean fresh water to the land — for free, over and over.

Without the water cycle, the land would have no reliable supply of fresh water, and life on land could not survive.

The water cycle is the most famous cycle, but it is not the only one. Many materials on Earth are recycled.

One important example is the way nutrients are reused. Nutrients are the useful substances, like minerals, that plants and animals need to grow.

Here is the idea, step by step:

• Plants take nutrients from the soil through their roots, and use them to grow.
• Animals eat the plants, so the nutrients pass into the animals.
• When plants and animals die, decomposers (like fungi and bacteria) break down their remains.
• This breaking-down releases the nutrients back into the soil, ready for new plants to use.

So the same nutrients are used again and again — a true cycle, just like water.

The big idea across all of these is the same: the Earth has a limited amount of material, so nature recycles it. Cycles are what allow life to keep going for millions of years without running out of the things it needs.

Understanding cycles helps with lots of other science topics:

• States of matter — the water cycle is a perfect example of solids, liquids and gases changing from one to another.
• Plants and growth — plants depend on both the water cycle and the nutrient cycle.
• Food chains and ecosystems — decomposers and the recycling of nutrients are a big part of how ecosystems work.
• Weather and climate — clouds, rain and the water cycle are the foundation of how weather happens.

Whenever you see something in nature happening again and again, ask yourself: is this a cycle? Spotting cycles is a powerful science skill that you will keep using.