WHEN I WAS A KID, someone told me that sunlight shining through a magnifying glass could burn a hole in a piece of paper. So, I went out into the sun and held a magnifying glass a few inches above a piece of paper—and sure enough, the light shining through the magnifier became so concentrated that it burned a hole in the paper. The action I took was a demonstration. The effect of that demonstration was the proof that sunlight shining through a magnifying glass is powerful enough to burn a hole in a piece of paper.
Dictionaries define demonstration in various ways, but first and foremost, as it applies to the field of science. One such definition says, "the act, process, or means of making evident or proving" (Webster's New World College Dictionary). In any science, demonstration is an action taken that proves something.
Think of the demonstration as the action you can take.
