Put a large pot of salted water on high heat. Peel and quarter potatoes. Boil potato quarters until soft, usually about 20-30 minutes. Be careful not to overcook but don't check them too often either - Mark Bittman advises that too much poking will cause the potatoes to absorb too much water. When done, drain and mash potatoes. For lumpy potatoes, use a fork or a potato masher, for smooth potatoes, use a food mill. Warm, don't boil, your milk in a pan over low heat, or in the microwave. Return mashed potatoes to pan over low heat, slowly stir in butter and warmed milk. Season with salt and pepper to taste!