Periscope

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A periscope is an optical device used to see objects above your line of sight. Basically, it consists of a tube with two parallel mirrors at opposite ends of a tube and at 45 degrees to the axis of the tube. Periscopes were used in the trenches in World War I, on submarines since World War II, and nowadays in gun turrets and tanks. It is easy to construct one for children out of basic materials.