Find someone gullible. Someone you like. Say:
"Do want to join the pen 15 club?
"It's a cool club."
Take a Sharpie, and ask your friend to extend a hand. Write:
PEN15.
It's best if you have the type of handwriting where it's hard to distinguish the number 5 from the letter S.
It will take a few showers before your friend gets the word "penis" off his or her wrist.