Plunge it
Use a flat-bottomed sink plunger, not a toilet plunger. Leave a little water in the basin, block the overflow hole with a damp cloth, and plunge firmly a dozen times. In a double sink, block the other drain.
Clean the stopper
Bathroom sink stoppers collect hair and soap. Most lift out or come free by loosening the nut on the pivot rod under the sink.
Open the trap
- Put a bucket under the curved pipe below the sink
- Loosen the slip nuts on each side by hand or with pliers
- Empty and rinse the trap, and check the pipe going into the wall
- Reassemble hand-tight, run water and check for drips
When professional help makes sense
If the trap is clear and the sink is still slow, the clog is in the line inside the wall. If other drains are slow too, the problem is further down and needs a drain machine.