It wasn't working. For some reason it was stopping instead of going backward, and Jet thought it might have been yarn that he'd untangled from some gears, something wrong with the control board (which he always kind of defaults to), or something wrong with the sensors in front.
I got the thing, took off my far glasses, and then picked it up. I studied the gears, studied the front sensors, tested to see that, indeed, it made the wheels stop when they were touched. Then I saw the wire on the back, and Jet said, "That can't be it, it's the sensor for when it's backing up."
"Oh?" I asked. "If it hits something with this wire does it stop backing up?"
"Yeah!" Jet said. "So that can't..."
it was amazing just seeing his face suddenly change as he realized what he was saying. "Oh!" He just lit up when he said, "Oh! So if it's always stuck ON....!?"
I just nodded, wiggled the wire, which had, indeed, gotten bent a little and wedged so that the contact was always 'on'. It popped loose. I handed it back to Jet, near-sightedly, and said, "Okay, try it now."
He ran off and I heard lots and lots of whirring, and then Jet yelled, "It worked, Mom! It worked! Wow... I really liked your reverse engineering, that's what it was, wasn't it?"
"Uhm... it might be more like debugging..."
We both giggled at the idea of debugging a bug, and I think it opened up a new way of thinking about things for Jet. That was very cool.