Gravity flow and telephone: what they have in common

1. Step 1. Gravity flow — is used in — Water filter. Water seeps through by gravity.
2. Step 2. Water filter — has part — Activated carbon. A charcoal filter.
3. Step 3. Activated carbon — is a kind of — Carbon. Porous carbon.
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.

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