Graphite pencil lead and heat pump: what they have in common

1. Step 1. Graphite pencil lead — is part of — Pencil. A mixture of graphite and clay.
2. Step 2. Pencil — is made of — Graphite. The lead is graphite mixed with clay.
3. Step 3. Graphite — is material for — Ion thruster. The grids are graphite or molybdenum.
4. Step 4. Ion thruster — has part — Solar panel. Power supply.
5. Step 5. Solar panel — is used in — Energy-efficient house. Solar energy.
6. Step 6. Energy-efficient house — uses — Heat pump. Heating.

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