I like the wallet example. Back in 2005, I found a wallet on the street with $3000 inside, and spent the rest of the day tracking down the owner. (Turned out it was a student's rent for her summer internship). Then maybe 2 months later I lost my wallet and a week later it showed up in my mailbox, cash and all -- someone took the time and their own money (as far as I could tell) to send it back to me.

And just to prove that I'm an irrational human, for the last 21 years every time I pass the spot where I found the wallet I can't help but look around for a lost wallet, as if it's some magical spot where the universe deposits lost wallets.