The Changing of the Guard - S3-E37
Continuity mistake: While at the window listening to the Christmas Carolers, the eyeglasses Professor Fowler is wearing disappear and reappear, depending on the camera angle.
Continuity mistake: When the contractor forces his way into Wanda's apartment, she faints, her head landing in the center of the bare floor. But in the following shot, as the contractor leans over her, Wanda's head is at the edge of a large throw rug. (00:15:30)
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.