Warm butter in a 6-8 inch omelet pan over low heat while you mix the eggs. Beat eggs, water, salt, and pepper vigorously with a fork or wire whisk for 20-30 seconds until well-blended and frothy. When butter is bubbly (but not brown), tip to coat sides of pan and pour in eggs. Turn heat to moderate high and, using a fork, draw edges of eggs as they cook in toward the center, at the same time tilting and gently shaking pan so uncooked portions flow underneath. Continue until omelet is just set and top is creamy and moist. Take the pan from the heat at once, loosen omelet edges with a fork, shaking gently. (If omelet should stick, slide a dab of butter underneath and tilt pan so it runs over bottom.) Fold omelet in half. Serve as is or top with tomatoes, mushrooms, or other savory morsels. Plain omelets can also be filled with almost any meat, fish, poultry, cheese, vegetable, fruit, preserves, or nuts. Meat and vegetable fillings are usually warmed in butter in the omelet pan, then topped with the eggs (by the time the omelet is cooked, the filling will be in the center). Cheese, fruit, nuts and other delicate fillings can be scattered over the omelet just before it's folded. Most fillings (cheese and nuts excepted) should be warm so that they do not cool the omelet.