I made my friend think I had burning hot ice in my mouth.
I bought this ice cup thing, basically fruit juice and water frozen in a plastic cup, that you hack at with a spoon for about 7 hours.
Anyhoo I decide to nuke it in a friends microwave and just drink the slushy remnants. But there was 1 chunk left so I shoved it in my mouth then spat it out and pretended it was burning hot - my super-intelligent science geek friend believed me!
Same guy, once he ran out of milk and was desperate for coffee, so I told him that mayonnaise was made from milk, like yogurt, and if he put a tablespoon of mayo in his coffee it'll taste fine, all creamy like. It didn't

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Same guy... We were flatmates at this point, and this is the last thing I did to him before he finally decided not to be my friend anymore. We found some 'Police, Do Not Cross' tape in the street, so I took some watered down ketchup and decorated his door to look like someone had bled all over it, sealed it up with the tape after drawing a chalk outline on his floor. Then we all hid when he came home from work so he'd think we were all out; I can't remember what he said exactly but it was funny.
Same guy... But my brother this time painted his face blue with permanent marker when he fell asleep drunk - he was annoying them all night with his fakery, exaggeration, and general douche-bagginess. So they smurphed him, then my little brother told him off for getting his sink covered in blue ink

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Now I feel I should explain, this guy who we kinda picked on is the sort of person who can walk into a room and make it cringe - not just the people in the room, but the actual structure of the walls cringe as soon as he makes an appearance. You ever met anyone who you want to tell to shut up before they even say anything. He got off lightly, well at least in these stories he got off lightly

- I'm not gonna tell you about the time when my 7 year old brother decked him and made him cry.