Anti-aircraft gun and greenhouse: what they have in common

1. Step 1. Anti-aircraft gun — is a kind of — Cannon. A gun.
2. Step 2. Cannon — uses — Casting. Barrels were cast.
3. Step 3. Casting — is used in — Window glass. The panes were cast in moulds.
4. Step 4. Window glass — is part of — Greenhouse. It lets light in and holds heat.

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