The homecoming dance
A somewhat socially awkward lad asked a girl he liked to the homecoming dance, and she said yes.
The boy asked his dad for advice, and his dad gave him several tips: get her a nice corsage that matches your boutonniere, show up 10 minutes early to pick her up and speak pleasantly and respectfully to her parents, agree on a return time that still gets you home in time for your own curfew, etc.
The boy said, "OK, I can handle that, but what should I do at the dance? I don't know any dances at all"
His dad said, "Don't worry: there's a total fallback for this. There's going to be a punch bowl at the dance. Every time a slow dancing song starts, just offer to refill everybody's punch. A whole bunch of other people will be there too and it will easily take the entire song's time to do, and you won't have to worry about dancing."
The boy got a nice boutonniere and matching corsage, showed up on time, respectfully charmed her parents, and got to the dance just on time. For the first few songs everybody was just relaxing and hanging out and he was having a great time.
But, eventually, a more romantic slow-dancing song comes on, and the boy nearly panics. But he remembers his father's advice. He says "let me go refill everybody's drink" and heads over to the punchbowl. But when he gets there, he realizes he is doomed, because, even after all that buildup, there was no punch line.