Abacus and freezer: what they have in common

1. Step 1. Abacus — is made of — Stone. Counting was done with pebbles — the Latin calculus.
2. Step 2. Stone — is material for — Ice house. Ice houses were built of stone and sunk into the ground.
3. Step 3. Ice house — evolved into — Refrigerator. The refrigerator replaced natural ice.
4. Step 4. Refrigerator — includes — Freezer. A refrigeration appliance.

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