Optical Illusion

Around the area known as More London, which is roughly half way between London Bridge and Tower Bridge, are these rather weird fountains. They are slabs of rock which at first glance just look like a nice place to sit, and yet the closer you get, the more you realise that if you sat on it you would get a very wet bum.

Don't ask me how it works, and how if you work nearby you don't feel the need to pee every 5 seconds.