That’s what the second hole in the bathroom sink is for. It has an essential role.

The second hole in your bathroom sink, known as the overflow hole, serves an essential purpose in maintaining the functionality and safety of the sink. Here’s why it’s there and what it does:
The Role of the Overflow Hole
1. Prevents Overflowing

The overflow hole acts as a safety feature. If the water level rises too high—whether due to a clogged drain or leaving the tap on accidentally—the hole provides an alternative exit for water, preventing the sink from overflowing and causing water damage.
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