Here's why:

* Density is a measure of how much mass is packed into a given volume.

* Water has a density of 1 g/cm³. This means that 1 cubic centimeter of water weighs 1 gram.

* Objects with a density greater than water will sink because they are more compact and have more mass packed into the same volume. This means they displace less water, and their weight pulls them down.

Example: A rock is denser than water, so it sinks. A piece of wood is less dense than water, so it floats.