The value of a particular stock is nothing but the perception of the value of the stock that the majority of the masses have. Okay, that was a mouthful.
Ref the tulip crises of Ireland
Or investing in art and painting.
Ancient paintings hold value and auction for such a high price simply because the common perception is that they are valuable. No one thinks the same of the paintings my niece made last summer.
The example of this idea being implemented and coming crashing down is a Ponzi scheme going bust or the dot com bubble or any financial crises for that matter.
People who once believed in the contrary, now learn en-masse that said item no more holds value it once did and is increasing decreasing