Heat flows from one object to another by conduction, convection or radiation, depending on the physical state of the medium through which it must flow. Heat always flows from a region of high temperature to a region of lower temperature. Without a temperature difference, there can be no heat flow.
Solids transfer heat through conduction. The object to be heated must be in contact with the heat source. Molecules closest to the heat source are heated first. They then transfer the heat energy to the rest of the molecules in the object.
Fluids transfer heat through convection. Fluid molecules that are heated up are less dense because they have a higher kinetic energy and their increased movement causes them to spread apart. Therefore, in convection, hot fluids rise and cold fluids sink.
Radiation does not require a medium to transfer heat. Heat flows as an infrared wave, which is a type of electromagnetic wave. This is how heat flows from the sun to the earth.
For heat to flow, there must be a temperature difference between the two objects involved in heat transfer. Temperature indicates the average kinetic energy of the molecules of an object. If the molecules have a high kinetic energy and move around quickly, the object has a high temperature. These molecules have the ability to transfer their energy to molecules with lower kinetic energy (colder molecules). Thus, heat flows from an object of higher temperature to an object of lower temperature until both objects reach the same temperature.