Why do things expand when they get hot?

Back to the atom!!

An atom will jiggle more the more energy it gets. Heat is energy and is carried by the PHOTON. A photon can cause an electron to jump to an atomic orbital of higher energy. The atom is now excited. An excited atom will have energy and wants to move. This is very much like a solid becoming a liquid, the solid receives heat and the atoms of the solid get excited, the solid melts as the atoms disperse or fly away.

Another way atoms can get excited is through friction. IE if you rub your hands together, the electrons get excited and your hands heat up. Your hand will EXPAND in the process, you can't visibly see this with your naked eye, but it does.

When our walking, your feet feel the rubbing of your footwear, your feet heat up, they expand.

The PHOTONS that we humans emit are of an INFRARED wavelength, you can't see them with the naked eye either.

Thanks, Adam. Much love. :-) X