Continuity mistake: When Ferris is in the phone booth, he tries to get the operator but then hangs up the phone. He then scans the Town Square. The camera pans counter-clockwise and the receiver is nowhere in sight.
Continuity mistake: The clock in the booth with a broken face is shown in a distant shot, then in a close-up, where more cracks appear in it than before.
Continuity mistake: When Ferris enters the cafe and announces they have a customer, he still has the stick in his right hand. He lays the stick on the counter as he begins to climb across. As the camera pans, and then back as he gets on the counter, a menu is now covering the stick.
Answer: It's explained in the episode. The aliens are experimenting to see how humans react to various scenarios. This just happens to involve them using super strength.
Thanks. It's been a long time since I saw the episode so I forgot.