OK do this:
1. Take a bowl(plastic not glass)
2. Piss in it.
3. Take a chair and a stick and drop the bowl up on the ceiling.
4. Tell ur friend u gonna do a trick with beer (if he asks.)
5. Ask him to hold the stick really quick so u can get something.
6. Move the chair and tell him to get it down
Watch and laugh as he spill ur piss all over him
7. Get new roomate xD
Can be done with water also but piss is soo much funnier xD
DEMO:
http://www.break.com/index/the-magic-stick-prank.html