Cash register and radar: what they have in common

1. Step 1. Cash register — has part — Keyboard. Sums were entered with keys.
2. Step 2. Keyboard — is part of — Mobile phone. A numeric keypad.
3. Step 3. Mobile phone — has part — Antenna. An antenna.
4. Step 4. Antenna — is part of — Radar. An antenna emits and receives.

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