This is a cool idea, a billboard in Germany has close shot of a woman's bust, her dark nipples can be barely seen through a white t-shirt, the copy reads "Guys, pray for rain." Here is the cool part, when it does rain, the t-shirt/ billboard gets wet and thus exposes more of the woman's breasts. The ad is for Playboy. More details can be found in the comment section of this website where one of the people who created the ad explains how they did it. Very simply, two shots were taken -- one with the t-shirt dry and one wet -- and when it rained the billboard switched to the second shot. It appeared in 2000 and seems to have won some awards. No word yet if passerbies tried to spray the billboard with water to get a glimpse!