A juxstapostion, whether internal (joke includes elements co-existing in an unlikely way) or external (subject or structure of joke creates tension between joke and common reality of listener) intended to arouse amusement.
a good joke:
"Two muffins sat in an oven. one muffin says to the other muffin:
"Boy, it's hot in here." and the other muffin says, "holy shit! a talking muffin!"
hahaha
What's wrong with same sex marriages? My wife and I have been having the same sex every Saturday night for over five years. (A good joke always has an element of truth).