Convinced a boy that boys have periods
In Year 9, there was a particularly gullible boy in my class called Weed. One day, the class clown and I decided to test the true extent of his gullibility.

Jon: So Weed, have you started your periods yet?
Weed: What the fuck? Boys don't have periods!
Jon: Yeah they do! Haven't you started having them yet?
Weed: Fuck off, boys don't have periods.
Me: They sure do, weren't you paying attention in science?
Jon: Yeah, if you haven't started yours yet, I'd be worried. You're supposed to start them when you're about 12.
Me: Yeah, what age did you start yours, Jon?
Jon: I was a late starter, didn't get mine till the end of Year 7
Me: Weed, how do you not know about this?
Weed: So..um...what happens?
Jon: It's horrible, all blood starts coming out your knob and stuff
Weed: Really?
Jon: Yeah, and for like a week beforehand all you can think of is chocolate and GBH
Me: Have you really not started your periods yet?
Weed: Oh, I just remembered - I have started my periods!
(grandmasterfluffles Come to my Armageddon party on, Fri 22 Aug 2008, 0:46, 5 replies)

South Park?
Ripped you off!
(Dangerous Disco Trev Isn't my real name, Fri 22 Aug 2008, 0:51, closed)

What????
When did South Park do it? Fuckers! Absolute fuckers! I so have copyright on this, it happened over 10 years ago. Bastards!
(grandmasterfluffles Come to my Armageddon party on, Fri 22 Aug 2008, 0:56, closed)

Wikipedia to the rescue
en.wikipedia.org/wiki/Are_You_There_God%3F_It%27s_Me,_Jesus
(Dangerous Disco Trev Isn't my real name, Fri 22 Aug 2008, 1:07, closed)

Eee-
vil!






Funny, though! :)
( Devil_In_Tights the *real* DiT!, Fri 22 Aug 2008, 10:26, closed)

I recall
Byker Grove did this as well, round about '93.
( Boss Keloid Got your mother's maiden name tattooed on my arm, Fri 22 Aug 2008, 15:58, closed)