Padlock and fitness tracker: what they have in common

1. Step 1. Padlock — is a kind of — Lock and key. A padlock.
2. Step 2. Lock and key — evolved into — Smart lock. An electronic lock.
3. Step 3. Smart lock — uses — Bluetooth. Bluetooth.
4. Step 4. Bluetooth — is part of — Fitness tracker. A link to the phone.

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