This time I convinced him to get outside the vehicle as I washed it (last time: he stayed inside and listened to a Sondheim musical). He had that huge spray nozzle thingie in his hands for a mere moment, and he instantly understood why Baba has so dadgum much fun washing the vehicles. And he was danged good at it, too.
I tried to convince him that going on a dump run was about as much fun, maybe more. “Huge piles of garbage, taller than that building there!” I said. “Ginormous tractors just pushing the garbage in a huge pile! So much flying dust they have to spray a mist of water down on the pile every so often! Can you think of anything more fun than that?”
As it happens, he could. Spraying the nozzle thingie with soap and water all over the side of the truck. I’m going to keep working on him about the dump run.