Adding heat to a liquid causes which of the following physical changes?
a. increases mass
b. increases density
c. decreases density
d. no physical change

In almost every case in nature, adding heat to a liquid
causes the density of the liquid to decrease.  That is,
when the liquid gets warmer, it expands and occupies
more space.

The one big exception to this rule is water ! 

Starting with a block of ice at zero°C (32°F), as the ice melts,
becomes water at zero°C, and all the way to 4°C (about 39°F),
its density increases all the way.   That is, it shrinks and occupies
less volume as it goes from ice at zero°C to water at 4°C.

This sounds like an interesting but insignificant quirk ... until
you realize that if water didn't do this, then life on Earth would
be impossible !