To understand how passive solar design works, you first need to understand how heat moves. As a fundamental law, heat moves from warmer materials to cooler ones until there is no longer a temperature difference between the two. Conduction is the way heat moves through materials, traveling from molecule to molecule. Heat causes molecules close to the heat source to vibrate vigorously, and these vibrations spread to neighboring molecules, thus transferring heat energy. For example, a spoon placed into a hot cup of coffee conducts heat through its handle and into the hand that grasps it.
Convection is the way heat circulates through liquids and gases.
Lighter, warmer fluid rises, and cooler, denser fluid sinks. For instance, warm air rises because it is lighter than cold air, which sinks. This is why warmer air accumulates on the second floor of a house, while the basement stays cool. Some passive solar homes use air convection to carry solar heat from a south wall into the building's interior. Radiant heat moves through the air from warmer objects to cooler ones.