Sketchpad and telephone: what they have in common

1. Step 1. Sketchpad — uses — Light pen. A light pen.
2. Step 2. Light pen — has part — Photodiode. A photocell catches the CRT beam.
3. Step 3. Photodiode — is used in — Fax. Scanning with a photocell.
4. Step 4. Fax — uses — Telephone. Transmission over a telephone line.

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