Have you ever had a habit that you kept to yourself, then one day realized that this habit actually has a name? And people (lots of people, maybe) actually do this thing too? When I was a kid, I always did anonymous little gestures for strangers. Things like, dropping drawings of eskimos into the supermarket freezer (drawn on paper snow flakes, of course).
I thought it was a bit odd so I kept it to myself.If you're keen, The Guerilla Art Kit by Keri Smith is a nice place to start. Or you could just get a pen and a scrap of paper, head outdoors and wing it.