What do you do when you see a space man?
Q: What do you do when you see a space man?
A: You park your car, man
I did another joke like this one before. I am not sure how I explained it, but at this moment, this seems like a pun that also has some grammar humor in it.
Addressing someone as "man" is a colloquial american english hippie subculture manner of speech, and the word "space-man" sounds like the words "space, man". The listener is most likely to hear "space-man", because that is a more likely phrase coming from most people's mouths.