This occurs when your 'victim' is taking a shower. This generally works better if the victim leaves the door unlocked. Act as if you are entering the bathroom to take a leak, brush your teeth, grab some mouthwash, etc. This will not work if you have a clear see through shower curtain. Grab a cat and get it so that it is very angry. Enter the bathroom with the angry cat, yell "NAPALM STRIKE!!!" and toss the cat over top of the shower curtain.
Johnny never locked the door when he took a shower. I thought I'd teach him a lesson by dropping a Napalm Strike. Ever since then, Johnny has locked the door when he showers.