Remembered, last part of dream: Jack was 'reducing'. It was one of those "I read something / watched a YouTube video that showed that you can do this, and now I want to do it, just to show that I can, because it's interesting" projects. He had some substance, maybe a powder, that he mixed into a cup or so of water. Then he poured that into a contraption that had 13 some-special-sounding-type-of magnets around the outside.

It was unclear to me what, exactly, was special about these magnets; were there 13 different magnetic poles rather than the usual two that we know about, and each magnet was oriented to put a different pole towards the inside of the cup? Or were they inducing regular old 2-pole magnetic fields, just in a special way?

In any case, the result was that, after Jack stirred up the water, a vortex formed in front of each magnet, with exactly 1/13th of the substance in each one. All he needed to do now was to scoop around one of those vortexes to have 'reduced' the quanity of the stuff by 12/13ths.

There may have been something else that was special or useful about the distribution of the particles; otherwise that seems like a very complicated way to measure out 1/13 of something.