Heat can transfer between objects in one of three ways depending on the medium: conduction, convection or radiation. Conduction occurs in solids, convection occurs in liquid or gas, and radiation occurs in virtually any medium, including a vacuum. The three methods of heat transfer have decreasing speed and efficiency, depending on the source and difference of temperature.
Conduction occurs when solid matter comes into direct contact with other solid matter. Heat is a function of kinetic energy; the molecules of warmer objects move faster than those of cooler objects. When a faster-moving molecule collides with a slower-moving one, they exchange kinetic energy and heat. The rate at which this occurs depends on the size of the objects and the temperature difference. For example, dropping an ice cube in a bowl of hot soup causes the ice cube to melt quickly.
Convection occurs in fluids. As a fluid heats up, its molecules are less rigid and less dense. This causes them to rise, displacing cooler fluid below, where it is heated up to form a convection current that carries heat from one place to another.
Radiation occurs everywhere, even in the vacuum of space. Every object radiates heat and energy, transferring it via electromagnetic waves; the hotter the object, the higher-frequency waves it emits. As a stove eye heats up, its emission frequency increases, until the wavelength reaches that of visible light. This is why the eye glows red as it heats up; an infrared video camera can detect the radiation even before it becomes visible to the naked eye. As it heats up, it eventually emits ultraviolet, gamma and X-ray radiation.