Adenosine triphosphate (ATP) is an organic chemical that transfers energy within cells. ATP is often called the "molecular unit of currency" of intracellular energy transfer. Energy cannot be created or destroyed, only transferred between forms. It is a property of objects that can be transferred or converted. Thermodynamics demonstrates how energy from the sun is transferred through grass, then a cow, and ultimately to a human consuming the cow. Muscular contraction, digestion, and tissue repair all require energy in the form of ATP.