The nucleus is in the middle of an atom. It is made up of protons and neutrons. Usually in nature, two things with the same charge repel or shoot away from each other. So for a long time it was a mystery to scientists how the positively charged protons in the nucleus stayed together. They solved this by finding a particle called a "gluon". Its name comes from the word "glue" as gluons act like atomic glue, sticking the protons together using the "strong nuclear force". It is this force which also holds the quarks together that make up the protons and neutrons.