Water in a radiator holds a lot of heat, but aluminum transfers it.

Aluminum is a much better heat conductor than water is. This fact is taken advantage of in radiator design. Aluminum radiators are filled with water, because water is excellent at holding heat. Then the aluminum’s high conductivity quickly transfers the heat from the water to the atmosphere.

Specific Heat

The "specific heat" of a material represents how much heat energy a given mass of a material absorbs before it rises in temperature by one degree. Water has a much greater specific heat -- more than four times as great as aluminum -- meaning that it can hold a great deal of heat energy.

Heat Conduction

Water absorbs heat energy very well, but aluminum is a much better conductor of heat than water is -- more than 300 times better. Many metals are especially good conductors because their outermost electrons are free to move about the entire metallic structure, forming a "sea" of free electrons. These electrons quickly transfer heat to cooler objects that come in contact with the metal.