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An hourglass is an amazingly simple device. It’s made of two glass bulbs that are placed on top of one another. The bulbs are either connected by a thin tube, or there is a very small opening in each of the bulbs. Sand is placed in the top bulb, and as the sand runs completely into the bottom bulb, a certain amount of time elapses. The sand hourglass can be inverted so that the sand runs back into the bulb.
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