I dumb joke my grandfather learned when he was a prisoner of war.

In a German prison camp, there isn't much to do. A new prisoner shows up, his name is Bill. One of the veteran prisoners, Jack, introduces himself to Bill. They talk, and eventually Jack asks what the prisoners do for fun.

Then a soldier yells, "15!"
Everyone is laughing hysterically.

Then another prisoner yells, "3!"
The laughter is deafening.

Bill asks Jack why these guys are laughing. Jack says that have been using the same jokes for so long that they save time by just assigning numbers to them.

Then a soldier yells, "7!"
Every other prisoner groans.

"Why didn't they laugh at that one?"
"Some people just can't phrase 'em right."