The CPU consists of many (i.e., 100 million to a few billion [ref]) tiny transistors built on a substrate. As the CPU performs its functions, electrical current is passed through the various CPU components leading to heat generation. This heat must then be transferred to the surrounding air, but must first make its way through all the materials in between. CPUs typically have a heat spreader to help transfer the heat to an object with more surface area first, before attempting to move it much further away from the CPU itself. Most often a heat sink is used, which consists of a typically metal object that has many extended surfaces (fins) that increase the amount of contact between the surrounding air and the hotter solid metal which is thermally connected to the CPU.