Thanks for the great question.

The water cycle shows how water changes from a liquid to a gas and then into rain or snow, and back again into a gas. Evaporation is when water changes from liquid to a gas. The gas then condenses to form clouds, and then rain or snow is called precipitation.

Scientists know about the water cycle by taking many careful observations of precipitation, how much rain or snow there is over the year, how much water is in lakes and rivers, and where the water flows. These scientists also measure the weather, for instance how warm the days are, to predict evaporation and condensation. These scientists, known as hydrologists, even use radar and satellites to help make these measurements. They also study physics and chemistry to predict how water will cycle from liquid to gas, and back again, through all the Earth’s environments.

We know how the water cycle works the same way we know how anything else works: people have studied it. Measurements of where water is deposited (e.g. where and how much rain and snow falls), where it goes from there (in rivers, etc.), where it collects (lakes, oceans), and where it evaporates from (bodies of water, as well as plants), allows scientists to make models of water patterns.

As a simple example, you can partly fill a jar with water and cover the top with plastic wrap. Mark the top of the water level. If you put the jar in the sun, a bit of the water will evaporate, and the water level will fall. Once enough water has evaporated, it will start to condense into droplets on the plastic wrap. If you tap the plastic wrap, the drops will fall as "rain" and raise the water level again.

Interesting question!

I think the best way to think about "how" we know about the water cycle is that scientists, or people who originally thought about if water was lost or gained every time it rained / water evaporated on earth, used the scientific method to eventually think about the problem.

If you know a little bit about how water evaporates, and how clouds make rain, anyone can begin guessing and thinking about ideas about what happens in between those steps. Then it took scientists in the 19th century to track water movement in rivers and other places to see if past ideas were right.

Check out the Wikipedia page on the water cycle and look at the section titled "History of hydrologic cycle theory" for a more detailed description!

water cycle

You can explore the water cycle on your own.

big steps are:
Going from a solid (ice) to a liquid. This is called melting.

Going from a liquid to a gas (water vapor). This is called evaporating.

Going from water vapor to liquid water. This is called condensation.

Going from liquid water to a solid (ice). This is called freezing.

You can melt ice and see what happens when snow and ice melt. If you live in a hot dry place, your water may come from snow and ice that melt in the mountains and flow down the hill to your town. How can you speed up melting?

If you take an empty container and set it in the sun, you can see evaporation as the liquid water “disappears” by becoming vapor. Does it matter whether the lid is on? Will it happen faster with more heat? Water gets from the Earth into the sky by evaporating.

You can take a container of ice water and set it in a warm room to see water vapor from the air condense into liquid water on the outside of the container. Does the temperature of the water in the container matter? When water vapor condenses on dust in the air, it forms clouds. When the droplets get big enough, they fall out of the sky as raindrops (or freeze to make snow, sleet, and hail).

You can put water in the freezer to see how water becomes solid ice. What happens to the volume (size) of the water? Does it weight the same before and after? Why does ice float on top of water?

Does ice ever turn into water vapor without being liquid water? Try putting an ice cube in the freezer for a long time (a couple of weeks). You can’t wrap it up, but maybe put it in a small container with a note asking people to leave it there. If you weigh or measure the ice first, you’ll get a more accurate idea.

Can water vapor freeze without becoming liquid water? The next time someone takes a package out of the freezer, see if ice forms on it.

Once you have done all of these steps, you will know a lot about the water cycle.