I created the Do Not Lick sticker in the summer of 1991. It's purpose is to warn people about things they shouldn't lick, like steam pipes, office desks, car tailpipes, high voltage boxes, &c.
Since its creation in 1991, these stickers have spread around a bit and shown up in a lot of places. The CMU KGB printed a more complex design at one point that was a bit more international.
Some people I know even dedicated a brick to this meme, which is located outside of CMU's Baker Hall. There was a newspaper story at one point that featured this brick prominently.
If you'd like to join the fun, I have some PostScript, and a PDF file for a page of stickers. They are meant to be printed out on sheets of stickers, 21 to a page (3 across, 7 down).