"In my opinion, JFK was the best president."
"Why?"
"He was very open-minded!"
"In my opinion, JFK was the best president."
"Why?"
"He was very open-minded!"
Alright listen up, buttercup. This joke is funny because it's a play on words. "Open-minded" usually means someone is receptive to new ideas. But in this case, it's implying JFK's head was literally "open" because he got shot in the head. You probably don't know what getting shot in the head is like because you have rocks for brains.