One of the things you get left with when making cheese is whey – milk separates in to curds (the solids) and whey (the watery stuff). It’s full of protein, and depending on the cheese you get quite a lot (from 2L milk for haloumi you get over a liter of whey), so throwing it out would be quite wasteful.
You can just drink it, warm or cold, as a protein drink. Since I’m not really a fan of drinking plain milk (and this is almost the same), so I was looking at other options. It can be used as an ingredient in lots of recipes, of course. Now, I still had some mango/banana/strawberry smoothie left over, so I mixed it with the whey (often people mix yoghurt in to smoothies but I really don’t like yoghurt) – and it’s an excellent mix! Super-tasty, and very healthy.