Have new theory.

Old wives' tale has it that a menstruating woman will make meat spoil, jelly or jam not set, and various other household tasks go wrong.

My theory is this. Having "everyone know" that these things just simply won't work does two things. First, it provides an excuse when a hot, miserable, menstruating woman standing in a hot kitchen goes into a shrieking, cursing fit and storms off to sit under a tree outside.

Second, it allows any menfolk or other womenfolk around to gently stop the cranky woman before she tries to do anything that's going to make her shriek, without having to question her competence or emotional stability.

I bet the original version went "a menstruating woman makes meat spoil, jelly not set, and dishes SPONTANEOUSLY break."

Yeah, that's it.

Did I mention I have a really bad case of fumblefingers and can't seem to knit for jack right now?