What happens when things get hot?
Heat is one of the most important forms of energy. When something warms up, the atoms inside it start to move more quickly, and they also spread farther apart. The hotter something is, the faster its atoms move. When atoms become really hot, they start to give out light. First they glow dull red, but if they get even hotter they turn bright red, yellow, white and finally blue. Heat is a bit like water, because it always flows 'downhill'. That is, it moves from warm things to cool things, but never the other way round. Things feel hot when some of their heat energy travels into your skin, and they feel cold when heat energy flows from you into them.