system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do springs originate at the top of the hills and how do they acquire such volume of water?

One explanation that I found was rainfall but it is not apparent that rainfall could contribute to such volume of water.
gpt
Answer: 

Think of a rock mountain actually more like a sponge. A sponge seems like it fills with water perfectly evenly, but when you zoom in if you were to fill a sponge certain holes would fill faster than other ones. There are little pockets of water in the side of the mountains. Sometimes the entire mountain is full of water, but oftentimes there's just a little pocket where it collects before pressure or gravity causes it to flow down to the next little pocket. A spring is just tapping into one of those pockets.