Step 2: The prank
Simply stated, fill each cup with water (about half-way is good) and place them close together on the floor of the area you'd like to obstruct. The closer together, the better. You should not be able to step between the cups (or at least not easily). Your prankee should find the set up and look upon it in awe, then quickly switch over to annoyance when they realize they have hundreds of tiny cups of water to dispose of to get to their bedroom/office/bathroom etc.