Definitive answers to the frequently-asked questions of /r/askscience.

If you post, please try to use this format:

1. Question in the title
2. TLDR answer
3. Link to detailed answer (on reddit or otherwise)
4. Post sightings of the question seen on reddit. To help keep the F in FAQ, only post comments you've seen appear multiple times; though you need not include every submission or mention.

If you can't provide a TL;DR, please copy-paste one of the better answers.