Ball cover and vacuum pump: what they have in common

1. Step 1. Ball cover — is part of — Ball. The casing.
2. Step 2. Ball — is made of — Natural rubber. The rubber balls of Mesoamerica.
3. Step 3. Natural rubber — is material for — Spray bottle. A rubber bulb.
4. Step 4. Spray bottle — has part — Check valve. A valve.
5. Step 5. Check valve — is part of — Vacuum pump. Valves stop air from flowing back.

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