If you like hear people screeming, this is the prank for you. Well you may not see the expression on his/her face but i'm sure that the sound is going to be loud enough. I want to apologise about the quality of some of the pictures but the light in the bathroom is poor.
Step 1: What You Need
Step 2: Preparing the Shower's Head
Almost every shower head is made in two parts -head and water ports and they are connected by carving. Between this two parts is where the "pills" should be. Unfortunately mine shower's head wasn't deatachable. But i untwist the hose's nut and put the "pills" inside the head. Then i put the head together and in place. Stay close to the door not to miss the scream!!