If you really hate your guests, looks no further than Jay Watson Design's thermochromic furniture. Before they sit down it looks like any normal old table and bench—but when they rise, their sweaty ass will leave a large bright patch that would make any man blush.
Made of solid oak, its surface is coated with thermochromic paint, so that when it's exposed to heat its color changes temporarily. Yes, just like those Hypercolor shirts you wish you could forget. At $1,850 it might be a little expensive for a practical joke—but then maybe you liked your Hypercolor all those years ago?More here.