A chain reaction starts with something, the instigator, so to speak, which may be an object or a person, animal, etc. It is followed by a series of incidents, that form the chain, and usually ends with some big event, or it could phase out slowly.. it may never end. The main thing to remember is that it is a chain, and all things that happen in it are linked. Any way..

Here's a good example of a real-life chain reaction that just happened today, in my crazy life:

My sister was being a jerk. I was trying to prepare dinner (spaghetti, for those interested).. I told my sister to get lost because she was driving me crazy. She in turn threw cheese in my hair. This was funny to me, so I threw the cheese back at her, attempting to get it into her hair. She then did it again and in order to stop the cycle I got up to play-wrestle/fight with her, and she took off out the door. At that point, she realized that this would only lead to me locking her out, so she tried to come back in as I was closing the door. The whole thing came to a pretty sudden end when she shoved her hand through the glass in the door window. Oh lordy. Needless to say, this wasn't such a good thing. (Also of note, this is the second time she's shoved her hand through the same door.)

The initial incident in this case was my sister, and the final incident was the smashing of the window. The cycle will of course continue, because we'll both get in trouble for breaking it, etc., etc.

There is also a moral to that story:

Don't smash windows, it gets you in trouble.