A substance floats or sinks in another because of its relative density.

Here's why:

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

* If an object is less dense than the fluid it's in, it will float because the buoyant force (the upward force exerted by the fluid) is greater than the object's weight.

* If an object is denser than the fluid it's in, it will sink because its weight is greater than the buoyant force.

Let me know if you'd like to explore this further!