In the very first episode of South Park, entitled " Cartman Gets An Anal Probe ", aliens plant a satellite inside of Cartman's ass. These aliens, colored grey with huge black eyes similar to the stereotype green alien figure we all know, appear in nearly every single episode of south park after the first episode at atleast one point in the show. You have to pay very close attention to catch it sometimes, and occasionally it'll be very vague, but a lot of the times they're mixed in with crowds of people. In the episode( I can't remember the premise of the episode ) where Cartman is learning to swim at the South park Pool and the kindergarteners continue peeing in it as he does, there is an Alien with a flotation device floating around the pool in one quick scene.
Also I forget which episode, but there is an alien sitting in the classroom amongst the children in one scene <_<
Also I forget which episode, but there is an alien sitting in the classroom amongst the children in one scene <_<