"Then he will tell his partner, and she will tell someone and then someone else will tell another and then one day at school, someone will say, let me tell you a joke - and I will hear my own joke.">

She's obviously a thoughtful person. People who know the joke will probably add their own thoughts to it. There will be a difference between the joke Smallsteps hears from his friends and the original joke. That might be the real surprise for him. Maybe she's curious about it.