find a wild farm land，prepare all this things:wood sticks,sweet potatoes,lots of clods( A clod of earth is a large lump of earth),
burn the clod to blcak even red！
amazing？when the clods are hot enough，push three or four small clods down into the clod house，and put the sweet potatos in it
then push the house down，use a hammer to crash the hot clods into powder，then cover lots of cold earth on,until it's not hot any more,for keeping hot inside the house.
What you want to do now is going home and wait for one and half hours.