Heat is the transfer of thermal energy between substances that are at different temperatures. Energy is always transferred from the warmer object (which has a higher temperature) to the cooler one (which has a lower temperature). Similarly, molecules with a lot of energy move faster than molecules with a smaller amount of energy, thus causing the former to have more heat. Heat transfer will continue until both objects have reached the same temperature or the same speed.
For example, an ice cube in a glass of water eventually melts. This is because the heat from the water, which is warmer, flows to the ice cube until both are at the same temperature, and therefore no ice cube is left.
There are three methods of heat transfer: conduction, convection, and radiation. Conduction occurs through direct contact. When two substances come into contact, their particles collide. The energy from the faster-moving substance is transferred to the slower-moving substance until they are moving at the same speed. At this point, their temperatures will be the same. An example of conduction is a spoon warming up when it is placed into a cup of hot cocoa.
Convection is the transfer of heat in air or a fluid through currents. An example is a pot of water warming up on a hot stove. As it heats up, the particles spread out and become less dense. The warm water on the bottom of the pot rises and displaces the cold water. As this occurs, the cold water sinks. So hot air rises, cools, and falls.
Radiation is the transfer of energy as electromagnetic waves. It does not need to directly touch anything or move particles as in conduction and convection. Radiation occurs through empty space, as in the sun heating the earth or feeling warm in front of a fire.
