Paper screen with grease spot and pneumatic tube: what they have in common

1. Step 1. Paper screen with grease spot — is part of — Photometer. The spot vanishes when both sides are equally lit.
2. Step 2. Photometer — is made of — Paper. A photometer compares the illumination of paper screens.
3. Step 3. Paper — is material for — Postage stamp. Paper.
4. Step 4. Postage stamp — uses — Postal service. Paying for delivery.
5. Step 5. Postal service — includes — Pneumatic tube. The post.

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