Hmmm - I've got a few.

My brother and I get my mom every year by Scotch taping the handle down on those hose nossles that are part of kitchen sinks - so when she turns on the tap it sprays her. She always forgets to check.

At my college, the bathrooms were co-ed, and set up so that the shower stalls were set behind the toliets - meaning, that if you climbed onto the toliet seat, you could reach your arm over into the shower stall. So one of my good friends and I would wake up early, fill up Britas with really cold water, climb up onto the toilet seats while people where showering, and he and I would dump the pitcherfuls on people. They never missed - it was so awesome. We got our entire floor last year.
Although to be fair, we got each other once, too. I don't think I ever screamed so loud at 7 am before.

My freshman year we conviced my roomate that the girl who lived next door was pregnant, and we were all going to chip in money to buy her an abortion. I still can't believe she believed us. :rolleyes2

A couple years ago, this really pervy kid at my college could never get a date, so he decided to plaster posters of himself around campus with his photo and phone number on it, asking for girls who were interested to call. He also listed his qualities, such as "funny" and "good problem-solver" on the poster. So a bunch of guys on my floor convinced me to call him. We figured everyone would win - we'd get a good laugh, and he'd get to talk to a real live girl. Anyway, we called from one of the guys' room, and I had a list of things to ask. I started with, "where would you take me on a date?" to which he responded, "I'm not really looing for a date, just a girl to hook up with on campus." LOL. So I asked, "If I hooked up with you, would you take me somewhere?" He said he didn't have a car. I said we could take mine, and asked if he liked movies. He said he liked the ones on campus cause they were free. What a charmer. Anyway, we went on and on about eveything from sex to meeting his parents to porn - it was so funny. We recorded the entire convo through the phone to one of the guys' computers, which lasted for over 20 minutes. I don't know how I held it together, but the tape of it has been played to so many people, I've met kids who go - "you're that girl!!!" Haha.