When you drop a toy into a bathtub full of water, it might go down to the bottom and stay there. This is what we call sinking. It’s like when a stone is thrown into a pond, and it goes straight down to the bottom because it is heavy and doesn’t float on the water.
Think of a big ship. If it gets a hole and fills up with water, it becomes too heavy to stay on top of the water and starts to sink. It’s like when you fill a cup with water; if you keep adding water, it will overflow. The ship can’t hold all the water, so it sinks.
Sometimes, things sink because they are denser than water. Imagine a sponge and a rock. If you put both in water, the sponge might float because it’s full of air, but the rock will sink because it’s solid and has no air inside.
This is how we understand sinking: when something is heavier or denser than the water around it, it goes down to the bottom.