Toothbrush bristles and bicycle: what they have in common

1. Step 1. Toothbrush bristles — is part of — Toothbrush. Tufts of nylon filaments.
2. Step 2. Toothbrush — is made of — Bone. Addis’s toothbrush has a bone handle.
3. Step 3. Bone — is material for — Ice skates. The first skates were horse bones with a polished side.
4. Step 4. Ice skates — evolved into — Roller skates. Skates for dry land.
5. Step 5. Roller skates — has part — Ball bearing. Bearings.
6. Step 6. Ball bearing — is part of — Bicycle. Ball bearings in the hubs and bottom bracket let the wheels and pedals spin easily.

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