Castile Soap

16 g lye (NaOH)
33 ml cold water or milk
120 g light olive oil
plastic water bottle

WARNING: lye is dangerous. Heed the warnings on the label. Use gloves,
protective clothing, and eye protection. Don't breathe the fumes. Don't use
aluminum utensils or containers.

Measure the lye and oil very carefully.

Put the olive oil in the water bottle. Put the water in a pyrex measuring cup. 
Slowly pour lye into water. Stir until clear. Pour lye+water mixture into
bottle with olive oil, and replace the cap securely. Observe that the water and
oil separates like Italian salad dressing. Shake vigorously until the mixture
becomes white and creamy (a couple of minutes). Periodically check for
separation and shake again. 

Once it has stopped separating, your work is done. This is called "trace". It
may happen right away, or it may take a couple of hours or even days of
intermittent shaking, depending on many factors. But if you weighed the
ingredients carefully it will happen.  Set it aside, out of the reach of
children, for 2 days. Then cut the plastic bottle and cure the soap in a dry
place for 2-4 weeks.