system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes the value of a stock go up or down?
gpt
Supply and demand.

There is a limited quantity of each stock.  When there are a lot of people selling but only a few buying, the buyers have more control of the price, so it goes down.  Conversely, when there are a few people selling and a lot buying, the buyers control the price so it goes up.