Some keys are easy to tell apart.  My car key, for example, looks strikingly different from my mailbox key.  No risk of mix-up there!  But what about that inevitable time when you have two keys that look exactly alike?  Do you Memorize the pattern of the keys' teeth?  Heck no!  There's an easier way...

...and you don't have to waste any nail polish or spray paint to do it!  Check it out: