Paper screen with grease spot e pneumatic tube: o que têm em comum

1. Passo 1. Paper screen with grease spot — faz parte de — Photometer. The spot vanishes when both sides are equally lit.
2. Passo 2. Photometer — é feito de — Paper. A photometer compares the illumination of paper screens.
3. Passo 3. Paper — é material para — Postage stamp. Paper.
4. Passo 4. Postage stamp — utiliza — Postal service. Paying for delivery.
5. Passo 5. Postal service — inclui — Pneumatic tube. The post.

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