Entropy, disorder, information loss, etc. Several times I mentioned that the Second Law of Thermodynamics is my favorite law of physics.
Eggs do not unbreak; candles do not unmelt; people do not unage; etc. The laws of dynamics (either classical or quantum mechanical) are time-symmetric, but why the Second Law of Thermodynamics isn’t?
Writing it in a more poetic way is also one of my favorite things to do: memories are of the past, never of the future, there, the arrow of time.