The tires. So fill the first one with soil and well rotted compost and poke your potato eyes in. The plants will shoot and grow. As they get taller, pop the second tire on and fill up a few inches more of rotted compost. The plant will keep growing taller to stay above the soil, and the plant grows the potatoes vertically. The tires protect them from the light, which is super important as green potatoes are really toxic. It keeps the potatoes contained and also means the harvest is much easier - no painful digging or potentially ruining your produce - just dismantle and remove your crop. :)