Bridges are structures that span physical obstacles like rivers or valleys to allow passage over them. The first bridges were made by nature, while early human-made bridges consisted of cut logs or planks. Bridges can be categorized by their structural elements, what they carry, whether they are movable or fixed, and materials used. Common types include beam, truss, cantilever, suspension, and movable bridges. Some famous bridges mentioned are the Brooklyn Bridge, Golden Gate Bridge, Tower Bridge, and Akashi-Kaikyo Bridge.