Why is my sink draining slowly, and how do I clear it?

Kitchen sinks block mainly from fat, grease and food. Bathroom sinks block mainly from hair, soap and toothpaste residue.

Steps to try, in order:

Clear the plughole of visible debris.
Pour a kettle of hot water with washing-up liquid down a greasy kitchen drain.
Plunge, covering the overflow opening first.
Remove and clean the trap (U-bend) under the sink.

To prevent recurrence, fit a plughole strainer, scrape food into the bin rather than the sink, and never pour cooking fat down the drain. Let fat cool and dispose of it in the bin.