Scientists now gathered information and facts before creating a theory.
Because of the enormous expense of creating conditions aboard
a spacecraft in which astronauts can survive, and the huge extra
weight and expense of guaranteeing their safety.
The hot molecules around the heat source expands, becomes less dense, then rises. When it rises, the cooler molecules moves down to take its place. This can occur in fluid, which include gas or liquid.
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I thought "Born Without a Heart" was pretty good.
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