Thermal Expansion and Contraction
Thermal expansion and contraction is how things grow bigger when warm and shrink when cool.
Think of it this way
A metal baking tray sits in a cold oven. The metal is packed tight, and the tray is its normal size. As the oven heats up, the metal bits spread apart, and the tray grows a tiny bit bigger. When it cools, the bits pull back together, and the tray shrinks to its old size.
Explaining thermal expansion and contraction by grade level
Wood gets bigger when it is warm. It gets smaller when it is cool. You can see this happen with wet wood. The wood grows and shrinks as it dries out.