Student is playing with a bouncy ball.
Me: Okay, thanks, that's mine now.  You can come get it at the end of the day.
Student: But what if I promise to play discreetly?
Me: You can play after 3:00.  Until then, it's mine.
Student: Aww man, you know I'm going to forget to come get it!  You just wanted to steal my stuff!