Thermal expansion is when things get bigger as they get hotter, and smaller as they get colder. This happens because when things get hotter, the particles inside of them start moving around faster and taking up more space. When things get colder, the particles start moving around slower and taking up less space. This can be seen in everyday life, like when a metal spoon left in a hot cup of soup gets hot and expands, making it harder to stir, or when a balloon left outside on a cold day shrinks because the air inside of it gets colder and takes up less space.