Water heater and cannon: what they have in common

1. Step 1. Water heater — includes — Solar water heater. A solar water heater is a boiler warmed by the sun.
2. Step 2. Solar water heater — has part — Window glass. The glass on top creates a greenhouse effect.
3. Step 3. Window glass — uses — Casting. The panes were cast in moulds.
4. Step 4. Casting — is used in — Cannon. Barrels were cast.

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