The water pressure inside my house is fine, but when I turn on the outside water faucet, the water pressure inside the house falls to zero. When I turn the water off outside, the pressure inside returns. What's up with that, and how do I fix it?

Are you on city water or pump? How old is your home? What type of pipes do you have, galvanized, copper, or plastic? Can you give me any more details? Back to you, Tom