Foam core and stone tool: what they have in common

1. Step 1. Foam core — is part of — Surfboard. A light buoyant core.
2. Step 2. Surfboard — evolved into — Snowboard. Surfing on snow.
3. Step 3. Snowboard — uses — Friction. It glides over snow.
4. Step 4. Friction — is used in — Quern. Grain is crushed and ground by the friction of the stones.
5. Step 5. Quern — is a kind of — Stone tool. A quern is two stones: the upper one is turned on the lower to grind grain.

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