Permanent wave and dewar flask: what they have in common

1. Step 1. Permanent wave — evolved from — Curling iron. A curl from tongs lasted until the next wash; a perm made it last for months.
2. Step 2. Curling iron — has part — Thermostat. A thermostat holds the set temperature so the hair is not burnt.
3. Step 3. Thermostat — is part of — Freezer. A thermostat.
4. Step 4. Freezer — has part — Thermal insulation. Polyurethane foam in the walls.
5. Step 5. Thermal insulation — is used in — Vacuum flask. Insulation.
6. Step 6. Vacuum flask — evolved from — Dewar flask. A household Dewar flask.

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