This is my favourite. I saw in a book once about new inventions of the 21st century and came across an article about lights that a person can put in toilets. It would shine a green circle onto the surface of the water if the seat was down, indicating to the ladies that it was safe to sit, and if the seat was up it would shine a red X, indicating that it was not safe to sit and also giving the guys something to aim at.
Question is, what if a person is colour blind?


















