Learn about the existence and sensation of heat, as well as its many characteristic properties.
Everything-toys, houses, people, plants-are all made up of atoms and groups of atoms called molecules. Molecules move slowly when cold and quickly when warm, which is the movement that produces heat. Hot air rises, which is how hot air balloons are able to fly. Hot water rises in an underwater volcano demonstration. When air gets hot, it loses density and will rise above cold water. Hot air molecules take up more space than do cold air molecules.