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A foundation is the lowest part of a building. A building needs a strong foundation if it is to stand for a long time.
To make a foundation, engineers dig a trench in the ground. They dig until they reach solid ground. When the trench is deep enough, it is filled it with strong, hard material. Sometimes concrete is used. Concrete can be made stronger by putting long thin round pieces of steel into the trench. When the concrete dries, the steel ties the foundation together. We call this reinforced concrete. Once the foundation has been packed down tightly, or dried hard, construction can start.