Nuclear fission is a process that splits atoms, specifically the nuclei of heavy atoms. It's not "made from" anything, but rather it's a process that happens *to* certain atoms.

Here's how it works:

1. The Right Atoms: Fission typically occurs with heavy, unstable atoms like uranium-235 or plutonium-239. These atoms have a large nucleus with many protons and neutrons.

2. Neutron Absorption: A neutron strikes the nucleus of one of these heavy atoms.

3. Instability: The absorption of the neutron makes the nucleus even more unstable.

4. Splitting: The unstable nucleus splits into two or more lighter daughter nuclei, releasing a tremendous amount of energy, along with neutrons.

5. Chain Reaction: The neutrons released in the fission process can strike other heavy atoms, causing them to undergo fission as well. This creates a chain reaction, sustaining the release of energy.

So, to summarize: Fission is not made from anything; it is a process that happens to certain heavy, unstable atoms.