There’s (or at least there was) a pile of fruit in the freezer. The freezer that is probably twenty years old and has noisily been giving up the ghost since we moved in, er, some decade-and-a-bit ago. Anyway. Jam.
Fruit into big pot on stove, four-fifths as much sugar by weight, boil until (in the words of my mum) ‘it all goes like boiling mud’. That is to say you’ve a pot of something about half the volume you started with, which is gobbing red hot sticky gubbins over all available surfaces. If it were just blackberries, I would have had to lob in a half-bottle of pectin, but since there were rasperries, strawberries and, er, other things in the mix, it all set very nicely of itself. You can, if you want, repeat the business with dripping a bit onto a cold plate every mumble minutes and seeing if it quickly forms a skin. However, once you’ve seen any jam or marmalade turn into boiling mud, you kind of know when it’s right.
… Which is one of those things that you don’t expect to learn.