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In the 17th century, scientists understood three phases of matter—solids, liquids and gases (the discovery of plasma, the fourth phase of matter, lay centuries in the future). Back then, solids and liquids were much harder to work with than gases because changes in solids and liquids were difficult to measure with the equipment of the time.
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Go Discovery! It was October 23, 2007 at 11:40am EST when I had my first ride to space on Discovery.