American and European household lore insists soap made on a waxing moon rises and sets properly, and soap made on a waning one stays soft.

It sits in a much larger family of rules — set eggs, sow leafy crops, cut hair for growth, all on the waxing half. The organising idea is simple and consistent: things you want to increase are begun while the light increases. Soap-making is chemistry and does not consult the sky, but the rule at least tells you what the tradition thought it was doing.