Unclog a sink without harsh chemicals

Try these in order. Most clogs give up at step 2 or 3.

1. Boiling water. Pour a kettle of it slowly down the drain in two or three goes. Works on soap and grease. (Skip this for plastic pipes that are already hot-water sensitive, or a porcelain sink with cracks.)
2. Plunger. Use a cup plunger, not the toilet one. Fill the sink with a few centimetres of water, cover the overflow hole with a wet cloth, and plunge hard 15 to 20 times.
3. Baking soda and vinegar. Half a cup of baking soda, then a cup of vinegar. Cover the drain, wait 15 minutes, then flush with hot water.
4. Clean the trap. Put a bucket under the U-shaped pipe below the sink, unscrew both nuts by hand, and empty it. This is where hair and gunk collect. Rinse and screw it back on hand-tight.
5. Drain snake. A cheap plastic one with little barbs pulls hair out of bathroom sinks easily.

Prevention: a mesh strainer in the drain and never pouring cooking fat down the sink.