Clinical Thermometer (or doctor's thermometer) is used for measuring a person's temperature. 

The temperature range that is marked on a clinical thermometer is about $35^{o}C$ to $43^{o}C$ (or $95^{o}F$ to $109^{o}F).$ whereas water boil at $100^{o}C$. That's why we can not measure temperature of boiling water using a Clinical Thermometer.