Rivers are like big long lines of water that flow through the ground outside. They form when rain or snow melts, and the water runs off and collects together into a big stream. Rivers can be really big or pretty small - some are so wide you can't see across them, and others are little enough that you could jump over them!
The water in a river is always moving, because it's trying to find its way to the ocean or another body of water. Because it's moving, it can be really powerful and move things like sticks and rocks along with it. Sometimes this can make the river look kind of murky because of all the stuff swirling around in it!
Rivers are really important because they provide water for people, plants and animals to drink and to use to grow. Lots of towns and cities are built near rivers so they can have easy access to water. People can also use rivers to travel on boats or to fish for food.
Overall, rivers are just really cool and important parts of nature that help keep the world a happy and healthy place!