In about 1900 it was theorized that the atom was composed of smaller particles. An atom is actually a system consisting of a cloud of negatively charged particles called electrons surrounding a nucleus, composed of positively charged protons and of neutrons, which have no charge. In the 1960s scientists discovered that protons and neutrons are composed of even smaller particles than first believed.

The term quarks came about from an American physicist named Gell-Mann. He borrowed the term from a line in James Joyce's 'Finnegans Wake' - "Three quarks for Muster Mark..." Subatomic particles come in many different types. The three main classes are leptons, quarks, and gauge bosons. The leptons are a class of particles that includes the electron, muon, and neutrino. They do not participate in the strong nuclear interaction, and they are thought not to be made up of quarks. The gauge bosons participate in the electromagnetic force. There is also said to be a graviton, which participates in the gravitational force.

Quarks are always found in combination and cannot, so far as known, occur by themselves. There are believed to be six types, called flavors. Quark flavors are now called up, down, strange, charmed, top, and bottom. The existence of the top quark is still being studied and has not yet been verified. Quarks can combine in many different ways forming up to 100 different particles. The combinations of quarks determine the type of particles formed. A proton is composed of two up quarks and a down quark. A neutron is composed of one up and two downs. It is believed that all matter in the universe is made of quarks and leptons.

Any particle made of quarks is called a hadron. A hadron made up of three quarks such as a proton or a neutron is called a baryon. A hadron made up of two quarks is called a meson. Less is known about how quarks are arranged inside hadrons. One commonly used illustration is called the bag model. A hadron may be though of as a tiny bag containing two or three quarks, depending on if it is a baryon or meson. Inside the bag the quarks can stay wherever they please, but the so-called strong force prevents them from staying too far from each other.

Quarks can jump from bag to bag in nuclear reaction, changing from one kind of hadron to another, but they can never be emptied. Some scientists suspect that quarks and leptons may turn out to have a structure to be made of yet smaller pieces. The six known quarks and leptons may turn out to be a final answer to the question, "what is the world made of?" They may also turn out to be yet another starting point.