Edge by Kevin Parker

Basically, rip a corner off a card and make a duplicate rip on another but don't take it completely off the card. Now take the fully ripped off one and attach it to a loop of IT with wax on both sides. One side of the it goes to the card, the other attaches to the first joint of your index (the part thats higher if you bend all your fingers back). Now you're ready.

Walk up to the window and show its solid (hiding the corner of the card with IT behind your hand) and then move that hand around the glass and stick the wax from your hand onto the glass. Now when you throw the card at the glass, hit with the ripped corner first and it will bend down behind the card. As this is happening, move your other hand (the one behind the glass) upwards so tat the the corner flys into view.

For the clean up, take the corner and show how it matches (do this on the opposite side as whichever direction the corner turned underneath) and rip off the corner still attached to the card as you do so.