Preheat oven to 400°F. Cut a cross on the potatoes. Put the potatoes at the top of the oven—straight on the rack, not a baking tray. Cook 1-2 hours (usually closer to 2). When they feel crunchy on the outside, pull the rack out, cut the potatoes open again to release the steam, then put them back into the oven. The major important thing is NOT to turn the oven down as they go soggy and lose their crunch. After ten minutes, serve immediately with shedloads of butter, salt and pepper.