To appreciate the feasibility of computing with no energy and no heat, consider the computational that takes place in an ordinary rock. Although it may appear that nothing much is going on inside a rock, the approximately 10^25 (ten trillion trillion) atoms is a kilogram of matter are actually extremely active. Despite the apparent solidity of the object, the atoms are all in motion, sharing electrons back and forth, changing particle spins, and generating rapidly moving electromagnetic fields. All of this activity represents computation, even if not very meaningful organized.