Water Kefir

The science behind the song

Water kefir grains aren't grains in the cereal sense. They're a symbiotic culture of bacteria and yeast, held together in a translucent, gelatinous polysaccharide matrix, that you keep alive by feeding them sugar water at a fairly narrow ideal range of around 20 to 24°C. Left to ferment, that culture converts sugars into organic acids, carbon dioxide, and a range of other metabolites, producing a lightly fizzy, tangy drink.

It's a slower, more hands-on relationship with fermentation than something like sourdough or kimchi: you're tending a living culture rather than just setting a process in motion. The "grains and strains" in the song are a fairly literal description of what's actually in the jar.