My favorite prank

Hmm... not sure how well this would work. Having actually thrown real grenades it is not as easy to pull the pins out as movies make it. Chances are real good that ^^ would just result in a broken line or the grenade hanging from the doorknob. For good reason it's supposed to take an effort to pull the pin and traps involving grenades usually already have the pins pulled, relying on holding the lever in place until tripped.