It's not rocket surgery

[Kuwagami]

http://6dollarshirts.com/t-shirts/Ro...t-p-12166.html

Guessing it's a pretty often misused idiom, like how often you see people spelling rouge instead of rogue, recieve instead of receive, effect instead of affect, your instead of you're (you are), their instead of they're (they are) or there (opposite of here) etc.

this was made for that