Every time it rains (I hear that whistle blowi- aaieeee stop stop) the words HELLO PEOPLE appear on the stair landing near my apartment as it dries. I assume someone painted this with Armor-All or some other sealant, so that the painted area sheds some of the water and dries first, while the texture and porosity of the concrete prevent it from actually getting slippery. One could gain a great deal of amusement by employing this on a large scale, bit by bit over time, on the walls and sidewalks of a city where it doesn't rain for a large part of the year...