Put a large pot of salted water on high heat. Peel and quarter your potatoes. Boil the potato quarters until they're soft, usually about 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 they're done, mash your 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 your mashed potatoes to the pan over low heat, slowly stir in the butter and warmed milk. Season with salt and pepper to taste!