Square root

A square root is the thing that you multiply twice to get a number. You know, like √2.

History
It goes back as far as the history of man.

Crazy or rational?
Well, any whole number that multiplies with itself and transforms into a whole number is a rational square root. Any other number is crazy. Put it in a mental hospital!

Square roots of negative numbers
Square roots of negative numbers don't exist. They must be a figment of your imagination.