Why is it than whenever presented with a hitherto unsighted diesel, if it has a prop attached, we can't resist the urge to flick it?

I do it as well.

A couple of years ago My PAW .35 was behaving like a bulldog that just ate a p*ss soaked thistle. It outright refused to hand start all afternoon. I took it home. I unceremoniously pulled it off the aircraft and swore never to mess with it again. Left it in on it's own in the corner - for corner time, like a naughty child.

Came back next day, picked it up, and casually flicked the little so-and-so.

Bad mistake ...

The mongrel thing fired and ran in my hand. I am sure it was only for second, but it scared the living wits out of me. My favourite female who happened to be standing nearby could not control her giggling, and muttered something about stupid men and their toys.