I first saw the one hand clapping koan posted in the window of a storefront on Haight Street in SF in the '60s. That and "Buy Free Things". I thought about those until my head hurt.
I've heard the tree falling thing but I was older then and knew enough not to let it bother me.

I ride off road singletrack bike trails through woods. I've done a lap and on the second lap a tree had fallen that wasn't down on lap one. If it had fallen on anyone it would've flattened them.

When your number is up ......