Matter is made up of atoms or molecules. These particles experience the effects of temperature in the form of agitation.

When temperature increases, most materials undergo an increase in their voume. This expansion, at the microscopic level, corresponds to a greater distance between the molecules. It is this increase in volume that we call expansion. Inversely, a decrease in temperature results in contraction. This property can be observed for all of the states of matter but it often isn’t visible.