Heavy ion collisions occur when large atomic nuclei, such as those of gold or lead, are accelerated to high speeds and smashed together. These collisions create extreme conditions similar to those just after the Big Bang, allowing scientists to study the fundamental properties of matter. In these experiments, researchers investigate the behavior of quarks and gluons, which are the building blocks of protons and neutrons. The collisions can produce a state of matter known as quark-gluon plasma, where quarks and gluons are no longer confined within individual nucleons, providing insights into the strong force that holds atomic nuclei together.