When I was young, and broke, an old guy bought my lunch one day when I was traveling. He left before I even got the bill, but the waitress told me he was a regular, and he left a simple note. And the note said something along the lines of "I was once young like you, and I know how scarce money can be when you are young. I hope that when you are older, that won't be a problem for you, and that you will pass along the favor"

Which I have done on a couple occasions, including the note, so they understood my motivation. I am hoping that when they get old, they'll pass it on.