Will the Real Martian Please Stand Up - S2-E28
Continuity mistake: During the scene in the diner between Ross the Martian and Haley the Venusian, there are objects such as a napkin dispenser visible on the counter over the left arm of Haley, which then disappear in the shots over Ross' right arm.
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.