1.) Vocabulary

Critical Mass: The exact amount of fissionable material needed maintain a fission chain reaction.

Fission: A nuclear reaction in which an atomic nucleus splits into fragments, usually two fragments of comparable mass.
View animation...

Fissionable Material: Material, such as the uranium isotope 235, that can go through the process of fission.

Fusion: A nuclear reaction in which nuclei combine to form more massive nuclei with the simultaneous release of energy.

Fission chain reaction: A chain reaction refers to a process in which neutrons are released in fission to produce an additional fission in at least one further nucleus. In other words, the neutrons/protons released in fission (or fusion) collide with other nuclei causing them to split and release more neutrons/protons and repeat the process, the domino effect.
Implode: When explosives are detonated on the outer surface of an object, instead of the inner surface, causing the shock wave to move inward, which crushes the object. This is the exact opposite of explode.

Subcritical: When there is too little fissionable material to maintain a fission chain reaction.

Supercritical: A term used to describe the state of a given fission system when the quantity of fissionable material is greater than the critical mass under existing conditions. Meaning that the amount of energy produced by nuclear fission is increasing rapidly, because there is more than enough fissionable mass, which usually leads to an explosion.

Uranium-235: A rare, and unstable, isotope of uranium that is capable of undergoing a nuclear fission chain reaction.