What prevents the mercury level in the glass tube of a clinical thermometer from falling on its own when its bulb is removed from the mouth of a patient ?

Explanation

**Step-by-Step Solution:**

1. **Understanding the Structure of a Clinical Thermometer:**
A clinical thermometer consists of a glass tube with a bulb at one end filled with mercury. The tube is narrow and has a scale marked on it to indicate temperature.

2. **Function of the Bulb:**
When the thermometer is placed under the tongue or in the armpit of a patient, the heat from the body warms the mercury in the bulb. As the mercury heats up, it expands and rises in the glass tube, indicating the patient's temperature.

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