Peel the potatoes and, with a very sharp knife, cut them into matchstick pieces, no more than about - in. (0.5 cm) thick. The easiest way to do this is to first cut the potato in half, crosswise (choose long potatoes so that you don't end up with short, stumpy fries). Place a potato half, cut side down, onto the cutting board and slice into 1/4-in. (0.5 cm) slabs from the top down. Lay each of the slabs flat on the cutting board and slice into matchsticks. Place in a bowl of cold water while you continue cutting the remaining potatoes. This, as you can probably see, is no job for a kid.
Drain the cut potatoes very well, then pat dry with paper towels. (Yes, kid job.) Place in a bowl and toss with the vegetable oil and salt, until evenly coated.
Spread out on a cookie sheet or wide, shallow pan so that the potatoes are in a single layer, not piled up on each other. Bake at 475°F (250°C) for 40 to 45 minutes, or until they're crisp and brown, stirring once or twice so that they cook evenly.