Ronald McNair, a physicist and one of the astronauts killed in the explosion of the Chaj’lenger space shuttle, held a black belt in karate. Here he breaks several concrete slabs with one blow. In such karate demonstrations, a pine board or a concrete patio block is typically used. When struck, the board or block bends, storing energy like a stretched spring does, until a critical energy is reached. Then the object breaks. The energy necessary to break the board is about three times that for the block.