Trouser legs and piston pump: what they have in common

1. Step 1. Trouser legs — is part of — Trousers. Two legs.
2. Step 2. Trousers — uses — Needle. Trousers were sewn with a needle.
3. Step 3. Needle — is part of — Syringe. A hollow needle.
4. Step 4. Syringe — works like — Piston pump. A syringe is a hand-operated piston pump.

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