This is one of the oldest tricks in the book, and a great way to bring a little hilarity into your morning routine if you have a roommate/house-mate/flat-mate/whatever. The premise is simple, sew shut the sleeve or sleeves (or leg or legs) of your roommate's favorite jacket/shirt/pants when they're not around.
Step 1: Materials
For this prank you will need:
-- An article of clothing for your target
-- A needle
-- Thread of a color similar to that of your target garment
Step 2: Execution
First, wait until your intended victim is asleep or absent for at least a few minutes, give yourself ten minutes just to have a little breathing-room. When you're sure to be undisturbed, thread up your needle with about a foot of thread (this is more than you'll need in most cases, but it's better to be safe). Knot one end of the thread, and then start sewing the sleeve (or whatever) shut. You should start sewing from the inside so that there isn't a telltale piece of thread sticking out on the outside.
Sew with a large running stitch, you don't want to permanently close the opening, just keep it closed long enough to cause a bit of consternation or confusion in your victim. When you've finished sewing, put the thread back through to the inside of the garment and clip off any excess thread to conceal what you've done as much as possible. Don't tie the thread off when you're done, that just ticks your victim off.