What is a kid friendly definition of upthrust?

Imagine you're in a bathtub full of water. You push down on the water, right? But the water pushes back on you! That push back is called upthrust.

It's like the water is trying to help you float! The bigger you are, the more water you push down on, and the stronger the upthrust. That's why big boats can float, but tiny pebbles sink.