[Almost] all liquids expand as they get warmer ... their VOLUME increases.
Since volume is the denominator of the fraction, that means
that when the volume INcreases, the density DEcreases.
So the liquid gets less dense when it gets warmer.
THAT means that if you have some liquid with different temperatures
in the container, the warm parts can FLOAT on top of the cold parts.
And THAT's what makes convection happen !
