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Chemistry
Why does rust make iron swell and crack?
The orange patch on a painted gate does not sink into the metal. It bulges, and the paint around it stands up in a blister that crackles under a fingernail.
Astronomy
How do we know what the stars are made of?
Nobody has ever been to a star, and nobody has brought a piece of one home. Yet astronomers will tell you what any star contains, in order, with amounts. The answer is hiding in the parts of a rainbow that are missing.