Number 8 on the Pronoia Therapy list:

You perform a senseless act of altruism, for instance by giving an anonymous gift or providing some beauty or healing to a person who cannot do you any favors in return.

I occasionally do the "pay it forward" thing at McDonald's drive-through. The reactions from the order-taker vary dramatically.

That is soooo cool!
Huh?
I'll have to ask my manager.
Nobody ever did that for me!! (Okay kid, here's your chance to be part of something positive for someone else!)

The reactions from the following car are funny to watch if I'm still in line. Some will honk and wave, some look like they're in serious discussion with the order-taker (probably trying to figure out what the catch is or if they know me), and some appear embarrassed.

Oops, now that I've posted this I've lost my anonymity. There goes the karma!