Card slots and fitness tracker: what they have in common

1. Step 1. Card slots — is part of — Wallet. Card slots.
2. Step 2. Wallet — is made of — Leather. Leather.
3. Step 3. Leather — is material for — Watch strap. Leather.
4. Step 4. Watch strap — is part of — Wristwatch. A strap.
5. Step 5. Wristwatch — evolved into — Smartwatch. The form factor of a wristwatch.
6. Step 6. Smartwatch — evolved from — Fitness tracker. Tracker functions moved into watches.

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