Entropy

tap reverse. try to believe your eyes.

mix two gases, then reverse every velocity — un-mixing looks fake, and is legal physics

Running every particle's velocity backward is completely legal physics — the gas really does retrace its steps. It looks impossible only because there are vastly more ways to look mixed than to look sorted, so un-mixing happens only when the odds get beaten on purpose; that imbalance of possibilities is what entropy actually measures.

Concepts: entropy, thermodynamics.

An interactive you play with one thumb, on Wundr.