Unblocking a sink or basin
Blockages are caused by food and other waste getting stuck in the u-bend or pipes. A simple plunger is the best way of clearing this. Don't use strong chemicals, especially caustic soda. If they don't work you will have a sink full of nasty liquid to deal with.
Remove as much of the water as you can from the basin by bailing it out with a cup. Seal the overflow by holding a cloth over it. Place the plunger over the plug hole and pump up and down quickly. This pushes and pulls the blockage apart. This should clear it.