Solar water heater and stone tool: what they have in common

1. Step 1. Solar water heater — has part — Window glass. The glass on top creates a greenhouse effect.
2. Step 2. Window glass — is part of — Wall mirror. Sheet glass.
3. Step 3. Wall mirror — is made of — Silver. Silvering.
4. Step 4. Silver — is material for — Syringe. Early needles were silver.
5. Step 5. Syringe — has part — Needle. A hollow needle.
6. Step 6. Needle — evolved from — Stone tool. Bone needles with an eye are part of the Palaeolithic toolkit.

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