Blood pressure monitor and air conditioner: what they have in common

1. Step 1. Blood pressure monitor — is made of — Mercury. A column of mercury.
2. Step 2. Mercury — is part of — Hydrometer. Old hydrometers used mercury or lead shot as the weight to keep them upright.
3. Step 3. Hydrometer — is used in — Lead–acid battery. A car battery's charge is checked with a hydrometer by the density of its acid.
4. Step 4. Lead–acid battery — is part of — Uninterruptible power supply. A battery.
5. Step 5. Uninterruptible power supply — is part of — Data centre. Backup power.
6. Step 6. Data centre — has part — Air conditioner. Cooling.

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