Mechanical pencil and tunnelling shield: what they have in common

1. Step 1. Mechanical pencil — is made of — Graphite. The lead is graphite.
2. Step 2. Graphite — is material for — Arc lamp. Carbon electrodes.
3. Step 3. Arc lamp — is part of — Street light. The first electric street lights were arc lamps (the “Yablochkov candle”, 1878).
4. Step 4. Street light — is part of — Tunnel. Lighting.
5. Step 5. Tunnel — uses — Tunnelling shield. Shield tunnelling.

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