The Most Useful Knot in the World (Taut-line hitch) #

This knot allows you to adjust its location on a string after you’ve tied it, and it stays in said location even under a lot of force.

See wikipedia for instructions on how to tie. Some of the many uses I’ve found for this knot:

Tying, then tightening, a clothsline
Tying drawstrings on sweatpants or shorts - you never need to untie/retie them again!