Continuity mistake: As the hour is striking midnight, Maggie's toy robot turns itself on and walks past the alarm clock on her bureau. After a cut to Lou and Mr. Death, the scene returns to Maggie's room, and the robot walks past the clock again. (00:22:00)
Continuity mistake: When Mr. Bookman is leaving his apartment to get away from Death, there is a note tacked to the door that is about level with his head. In the next shot, he's out in the hallway locking his door and the note has moved up to about 6 inches above his head. (00:12:35)
Continuity mistake: Death has a bag full of purchases made from Lou. As they walk off, Death's bag disappears.
Continuity mistake: In the opening, Mr. Bookman has a toy replica of Robbie the Robot in his left hand in the close up, but a bottle in it a second later.
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.