Pencil and heat pump: what they have in common

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

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