Learning how to create fire was pivotal in human history, providing our ancestors with warmth, protection, and a means of cooking food. It wasn't until the early twentieth century that we discovered that heat is not a substance. When popcorn is heated, energy flows into the kernels and the liquid water molecules inside move faster and faster, turning to steam and popping the kernel shell. As atoms and molecules take on energy, they speed up; the faster they are moving the hotter the substance becomes. Heat energy is movement, and has the ability to transform even matter itself.