The small opening near the top of a bathroom sink is called an overflow hole. Although it may look unnecessary, it serves as a backup drainage passage when the sink begins filling too high.
If the tap is accidentally left running while the drain is closed or blocked, water enters the overflow opening before reaching the rim. A hidden channel inside the basin directs that water into the main drainpipe. This can slow the rising water and may help prevent the sink from overflowing onto the floor.
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