This is caused by a build up of fat, tea leaves, hair etc. in the waste pipe. Try pouring hot water down the plughole, or flush through with proprietary cleaner or water containing caustic soda crystals.
If this fails then:
- Empty the bowl or sink. Bail it out with a cup or jug.
- Block the overflow opening. Place a rag tightly over it.
- Clear the blockage. Place the plunger over the plughole and work forcefully until the blockage clears.
- Clear out the u-trap situated immediately below the plughole. Place a bowl under the trap, unscrew the joints and remove the trap – then clean and re-fit, checking that the seals are in place and that the joints are screwed up tightly.
If the above procedures fail to unblock the sink you will require the services of a plumber.