Hektore

I second both of these, you can line the underside of the door and just use a small fan to blow the powder under. If you want much more fun than baby powder, go to the hardware store and get chalk dust, the kind you would put in a chalk line. It's not just a pain then, it's also hot pink, neon orange, bright blue, whatever your taste may be. I'm also a big fan of bricking up doorways. I don't know how much time you spend at their place, but there is nothing quite as classic as short-sheeting someone (turn the sheet so under the cover, it lays long ways across the bed rather than down the length).

//edit: more ideas. Speaking of cars, just unhook a battery cable, or spark plug wires (this works best if you take them out and leave them on the seat). You could also disconnect the shifter cable, which leaves the car in park until fixed. Another great idea, just throw a condom into one of their beds. Better still, randomly leave a positive pregnancy test somewhere they'll both see it but it won't be in the open, so they have no reason to move it. That way they'll both assume it's the other one. Shut off the water heater.

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The advantage law is the best law in rugby, because it lets you ignore all the others for the good of the game.