A centrifuge is a machine designed to separate heavy material from light material. When the substance is a very fine solid in a solution, separating parts of the mixture can be more difficult than a solid. In our case, we will use a centrifuge to separate DNA from a liquid so we can focus just on the DNA. A centrifuge works by spinning the substance at high speed. You have probably seen the trick where you fill a bucket of water about halfway full, spin it around, and watch as the water stays in the bucket. The water is pushed against the bottom of the bucket by what is colloquially known as centrifugal force. Though not a real force, but a reaction to the centripetal force, centrifugal force pushes against the bottom of the bucket. This causes the particles in the solution to clump at the bottom of the tube. That solid clump is called a pellet, and the solution above it is called the supernatant .