Don't remember where I saw this demo (perhaps the Ontario Science Center?) and have not done it myself, but the idea is to take an empty milk jug and heat one spot with a heat gun until the plastic melts but stays intact. The cloudy crystalline high density polyethylene - HDPE - turns clear during the heating. Then, blow into the jug to expand out that wall. When it cools, it will harden and turn cloudy again. This is a nice demonstration of plastic behavior and blow-molding.
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