You might have felt that your ears have 'popped' when you were in a plane taking off or in a car travelling down a steep mountain road. Why did it happen?

Explanation

### Step-by-Step Solution:

1. **Understanding the Situation**: When you are in a plane taking off or in a car going down a steep mountain, you experience a change in altitude. This change affects the air pressure around you.

2. **Air Pressure Changes with Altitude**: As you ascend in a plane, the air pressure decreases because there is less air above you. Conversely, as you descend, the air pressure increases because there is more air above you.

3. **Effect on the Eardrum**: The human ear has a structure called the eardrum, which is sensitive to pressure changes. When the external air pressure changes rapidly, the pressure inside your ear may not equalize immediately with the external pressure.

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