What causes geysers to erupt?

I'm doing a 5 page research paper on the geysers of Yellowstone National Park. Part of my paper is on why Old Faithful erupts. Another part of the paper is just on different geysers. One of my questions is "How does Old Faithful erupt every 90 minutes & what causes it to erupt"? My other question is "What exactly causes the geysers to erupt"? I have about 15 note cards so far on it. But It's just mainly facts right now. And so now I just need to get into the facts about the eruptions and what causes them. If someone could help me. Thank would be fantastic. Thanks a bunch :)