system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did people get fountains to work in medieval times?
gpt
Gravity, water and air pressure, and an understanding of fluid mechanics (especially siphoning).

If you have a body of water (a little pool, for example), and a very narrow tube leading from the bottom of the body of water to a position that is higher than it, the pressure from the pool will force the water up the tube (so long as there is no air in the tube; this is called siphoning). 

So you begin the siphoning process manually (sucking the air out of the tube), then install the tube into the fountain. So long as the pool of water never goes empty, you never have to restart the siphoning. It'll just keep going forever.