Entropy (arrow of time)

Entropy is a measure of how much disorder or chaos is in a system. The arrow of time is a way of describing why time seems to always move forward and never backward. The arrow of time is closely related to entropy because, as time passes, entropy tends to increase - meaning the world, and everything in it, moves from a state of order to a state of disorder. For example, a new glass of milk is in a state of order at the beginning but, over time, it will become stale, which is a state of disorder. Note that entropy doesn't have to do with the time that passes, but with the changes that occur in the system and how those changes increase the amount of disorder.