These guys had all been in prison together for a long time and a new inmate arrived.
When it was night and lights out, someone said, "16" and everyone cracked up. Someone else said "23" and people cracked up again. Someone else said "45" and everyone was rolling.
The new guy asked his cellmate what was going on and was told that they'd all been together so long, and knew all the jokes that instead of reciting the whole joke they'd numbered them.
Then someone yelled out "29" and nothing happened, no response. Again, "29" and no reaction. So the new guy asked his cellmate what was up and he said, "Well, you know, some people just can't tell a joke."