> which when I was a wee child made no sense to me because "surely then nothing would happen"

I've literally just had this conversation today about jet aircraft with my 5-year-old, and how when you push air out of the jet engine really fast, that action pushes the nozzle forwards - it's not pushing against anything, it's like if you push against the sides of a box.

But he's still not quite got it, so we're going to go to the workshop and play with a high-volume fire pump.