Tantalus Cup, A self-siphoning cup that can never be filled. The Tantalus Cup helps demonstrate Pascal's principle, or how a siphon works, Each

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A glass cup with a U-tube attached When water reaches the top of the inverted U-tube, a siphon action begins which drains the water until the end of the U-tube is exposed When the cup is filled with water, the water is forced up the inverted U-tube so that equilibrium is maintained in the cup As the water level reaches the top of the U-tube it is begins to fall through the siphon As the water falls, it creates a vacuum which causes more water to be forced into the tube