I watched them pour the driveway to our house. The workers laid down steel rods, then as they poured the cement, they pulled the rods up so they would be in the middle of the concrete as it hardened. "What do you need the rods for?" I asked one of the workers. "It makes the concrete stronger. Reinforced concrete." "Yes, I know, but how do the rods make the concrete stronger?" The worker picked up one of the rods. "Look, if you push down on it, it bends real easy." His muscles bulged and the rod bent. "But you couldn't pull it apart. This hunk of rod could pull that truck over there. On the other hand, a piece of concrete is easy to pull apart. But if you push down on it, it won't bend." "So?" "So they've got opposite strengths. The steel is strong when you pull, the concrete is strong when you push. Put them together, and you've got reinforced concrete, which is strong both ways. That's how they make all those big buildings and bridges. Concrete by itself or steel by itself wouldn't be strong enough."