There are two answers to this:
- Its density decreases
- Its volume increases
You notice that ice floats on water, this means that its density is less. The mass remains the same because you did not add or remove anything. But what causes the density to decrease then? For density to change, there needs to be a change in either the volume or the mass of the object. Density is calculate by mass/volume. Since the mass did not change, the volume must have. So in this case, the volume increases. Hence, this is what causes it to be less dense.