People are already using NeverWet (aka the silicon-based, water-repellent spray that's been getting all the buzz lately) to waterproof just about anything that can't squirm away. But here's an interesting take from two members of Home Depot's "How-To Community." Nathan Sharratt and Dana0814 made some stencils for NeverWet sidewalk graffiti, which can be seen only in rainy weather. It's just a matter of time (and probably not much time) before advertisers will be all over this idea—and in fact, similar things have been tried in the past. But it might not go over well in every city.