"Arrow?"
"I tell them it's your secret superhero name."
Near enough - it was my now-declassified code-name back in the day.
"Retired superhero," I correct him. "You've got two retired superhero grandfathers, Arrow and Jedda."
(Grandfather Karel is actually "Děda" which is Czech for 'Grandpa' but sounds as if it belongs to the same word group as 'Jedi' with its heroic overtones.)
With a suitably serious and thoughtful expression I say, "I wonder what your secret superhero name will be when you're bigger?" (I come from a proud line of straight-faced story-tellers and yarn spinners). "I guess you'll find out when the time comes."
"Anyway", I say, "what were you going to ask?"
"What happened to your arm?"
Suddenly I realise that although he was right in the middle of the mêlée, he is the only person who didn't actually see what happened. One second he was running around having fun, next second he was on the ground in a tangle of arms, legs, hosepipe and people screaming.
I say, "Remember when we were having a water fight yesterday, and chasing each other with the hosepipe?"
"Yes."
"And then we all got tangled in the hose and fell down?"
"Yes."
"Well, we were running very fast and were going to fall into the wall, so I put my arm out to stop us crashing into it. But we hit it so hard my arm broke."
He thinks about it for a moment, then his eyes widen.
"You sacrificed your arm to save us!"
For a moment I think perhaps I should unwind this a bit. But then I speculate on what he is going to tell his classmates when they return to school next year and are asked to describe what happened in the holidays.
"My superhero granddad Arrow sacrificed his arm to save me."
I think we can live with that.