A water-soluble substance is something solid that disappears into water when you mix it in. It does not just break into smaller pieces and sit at the bottom — it truly spreads out through the water and you cannot see it any more. Sugar and salt are the classic examples: put them in a cup of hot water, stir, and they are gone, though the water now tastes sweet or salty, so you know they are still there.

You meet this word most often in three places. In science class, where things are sorted into water-soluble and not water-soluble (oil is the famous one that refuses). On medicine and vitamin packets — vitamin C and the B vitamins are water-soluble, which matters because your body cannot store them well; extra amounts leave in your urine, so you need them regularly. And on product labels: water-soluble ink, water-soluble glue, water-soluble fertiliser, water-soluble packaging for laundry pods. There the word is a promise about convenience — it will wash out, it will dissolve on its own, you will not have to scrub.

The opposite side has two words. `insoluble` means it will not dissolve at all. `fat-soluble` (or `oil-soluble`) means it dissolves in fat or oil instead of water — vitamins A, D, E and K are like this, and that is exactly why those ones CAN build up in your body. The word is a little formal and technical; a friend would say 'it dissolves in water', and only a label or a teacher says 'water-soluble'.