A tidal stream generator is a machine that is put into the ocean and uses energy in the motion of the tides to create electricity. Basically, when the tide moves, it makes the blades on the generator spin around, like a windmill does with wind. This spinning creates electricity that can be used in homes and buildings. So, when the tide comes in, the blades spin and make electricity, and when the tide goes out, the blades also spin and make electricity.