Nuclear Fission is an amazing process. With a fuel that is inert (even enriched uranium won’t burn in a flame), the input of a single neutron can cause the release of more energy than tonnes of explosive.
How does a single neutron do this? A single neutron initiates a reaction which causes the release of more neutrons, each of which can initiate further reactions. This process is called a chain reaction, as the individual events in the reaction are caused by previous events.
You can envision a chain reaction as a snowball rolling down a mountain, picking up more and more snow until an avalanche is unleashed. You can model how a single change can cause a cascading sequence of events by playing this game (Can you beat my top score of 1170? Provide a screen shot if you expect me to believe you!).
So what does a chain reaction look like? Well, because of the size of the particles involved, the speed and energy of the reactions, we can’t actually see the individual events (fissions). If you want to see what it looks like, check the Tsar Bomba update on the Boomology post. We can theorise what they look like from what we know how they work, and we have various simulations: