Trolleybus and electrical telegraph: what they have in common

1. Step 1. Trolleybus — has part — Pneumatic tyre. Runs on tyres.
2. Step 2. Pneumatic tyre — has part — Tyre tread. A tread.
3. Step 3. Tyre tread — is made of — Carbon. With carbon black for strength.
4. Step 4. Carbon — is material for — Hearing aid. The first hearing aids used a carbon microphone.
5. Step 5. Hearing aid — evolved from — Telephone. The first electric hearing aids were built from telephone components.
6. Step 6. Telephone — evolved from — Electrical telegraph. The telephone grew out of attempts to send several signals over telegraph lines (the “harmonic telegraph”).

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