Dewar flask and telephone: what they have in common

1. Step 1. Dewar flask — uses — Vacuum. A vacuum.
2. Step 2. Vacuum — is used in — Triode. It works in a vacuum.
3. Step 3. Triode — is part of — Amplifier. Tube amplifiers.
4. Step 4. Amplifier — is part of — Hearing aid. An amplifier.
5. Step 5. Hearing aid — evolved from — Telephone. The first electric hearing aids were built from telephone components.

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