Let's say you drop a glass of milk and the glass smashes into 27 pieces and milk spills all over the floor. If you videotaped this and ran the film backwards it would be obvious to you that the film was running backwards. Why?

The process violates:

1. The Law of Conservation of Energy (1/32) (3%)
2. The Law of Conservation of Momentum (1/32) (3%)
3. The Second Law of Thermodynamics (Entropy Never Decreases) (21/32) (66%)
4. All of the above (9/32) (28%)

Only the Second Law of Thermodynamics would be violated - this is why entropy is sometimes called time's arrow. Time moves in the direction of increasing entropy.

This is an example of an irreversible process. A process is irreversible if energy is lost to friction, or if energy is lost as heat flows from a hot region to a cooler region.

A reversible process is one in which there is no change in entropy, and the system and the surroundings can be returned to the initial state.