There are common super tiny living things called Giardia that are kind of like turtles. When they see a dangerous environment they hide away in their shell that they call a cyst. When you freeze the water the little Giardia just hang out in their shell which is insulated like a cozy cabin. But when you boil water it gets too hot in the shell so the Giardia have to leave their cyst/ Shell where they are exposed and then killed.
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