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 arse 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 colour changes temporarily. Yes, just like those hypercolour shirts you wish you could forget. At $US1850 it might be a little expensive for a practical joke — but then maybe you liked your hypercolour all those years ago?