One-lightenment: one-line enlightenments (personal ones & ones I have learned from others)

Caveat Emptor: some enlightenments might not be a one-liner; it might be more than one line, or less than one.

(Vladimir Lenin) All extremes are bad.
- The idea of balance appealed to many philosophers, both eastern and western ones. See the Confucian school of thought and Aristotle’s Ethics of “The Golden Mean”
(Sartre) ‘Happiness is not doing what you want but wanting what you do.’

The less you are known to the world, the more you can do.
(Richard Feynman) I would rather have questions that can’t be answered than answers that can’t be questioned.
(Unknown) Be careful what you wish for, because you may get it.
- I’d like to attribute this to St. Augustine, but it could be that many saints and non-saints shared this noble thought.

(Unknown) “For sale: Baby shoes. Never worn.”
- Popularly attributed to Ernest Hemingway, but the exact attribution is unknown.
Everything is a social construct! (joke among sociologists)
(Homeomorphism) https://en.wikipedia.org/wiki/Homeomorphism. A coffee mug and a donut are, homeomorphically, the same!
- A mind-blowing realization I had in real analysis class.