Teach me something obscure yet somewhat useful that you know?

I want to learn something new, and am asking you for help. Hopefully this can be a chance for folk to share some interesting knowledge. I’ll try to chime in when I wake up in the morning.

For a little more guidance, I welcome academic or applied knowledge. Conspiracy or overly religious is not really welcome.

For example, a religious fact like: Jesus had 4 brothers named X, X, X, and X is okay, but something about how you can see Jesus if you look at your mom’s bathroom picture is not.