Black holes were first predicted by John Michell in 1738. The smallest black hole can be 1/100 cm (3937008/1,000,000 In) with the mass of the moon. The largest one can be 1.5 billion Km (932 million mi) with the mass of 1 Billion suns. How a black hole forms is by a super massive star collapsing. When a star starts to die, its gravity becomes too powerful and the star starts to implode upon itself. When you fall into a black hole you go through a process called spaghettification which is basically when the gravity on either side of your body is so different that your body starts to pull itself apart ( I would say probably not the most pleasant experience). In a black hole there is something called the event horizon. When you fall into a black hole, to somebody watching outside the black hole you appear to get stuck on the event horizon.