system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does water come out of the tap as soon as you turn it on?

How does it come out so suddenly? Wouldn't the water have the travel from where it comes from all the way to the tap? Is there some sort of reservoir beneath the tap?
gpt
There's always water in the pipe. All the way to the water tower or whatever the source of your water pressure is. When you turn off your tap, that water that almost made it through? That's the first water to come out of the tap the next time you turn it on, whether it's a minute or a month later.

The whole system is fed by gravity. The source of the water is at a higher elevation than you are, so anytime you open the tap, the water will flow because of gravity, even if the water between the tap and the source has to go below your feet on the way to you.