Excellent video on how to unclog a bathroom sink
by Mark Frauenfelder from on (#1A8D9)
https://youtu.be/Bu3SDZSGlxo
The folks at This Old House use clear plastic plumbing to explain how bathroom sinks work, and how to unclog them.
We live in an old house and the pipes get clogged a lot. My favorite bathroom sink unclogger is the Drain Weasel.