Black holes are stars that have collapsed into one very small point. This small point is called a "singularity".This singularity is a point of space-time which rotates at a high speed.That is the reason that black holes have no time. Black holes suck things into their center because its gravity is very strong. Some of the things it can suck in are light and stars. Only very large stars, called "super-giants", are big enough to become a black hole. The star must be one and a half times the mass of the sun or larger to turn into a black hole. This number is called the "Chandrasekhar limit". If the mass of a star is less than the Chandrasekhar limit, it will not turn into a black hole; instead, it will turn into a different, smaller type of star. The boundary of the black hole is called the "event horizon". If something is in the event horizon, it will never get out of the black hole.