by Jodi Lee Reifer

Friday May 16, 2008, 1:00 AM

As any know-it-all can attest, tap water comes from the faucet. Duh.

But true trivia-hounds know NYC's H2O is collected in the Catskills Mountains. One of the spots is called Neversink Reservoir. No joke.

The American Museum of Natural History leads a bus trip up there Saturday for a behind-the-scenes tour of the water works. It comes in conjunction with the museum's "Water: H2O = Life" exhibit.

Meet at 8:30 a.m. on the museum steps at Central Park West at 79th Street, Manhattan. The bus returns at 5 p.m. Fee: $25. Lunch included. Reservations necessary. 212-769-5100, http://www.american-mnh.org.