You just have to purchase a telescope, then place it in your yard. Then stargaze at around
10 pm at night, wait until a light beam shines on your Sim and your Sim will, like, float
away then you will hear it scream and a few techno noises. Just fast forward, then your
Sim will return then he/she will go to the toilet to puke. Not only that she/he will have
a new pet treat called Mood Enhancer. But when you will feed your pet with that treat it
is called Alien.

Hope that helped. :D