The Magic Potion would be made from a hlaf-empty yoghurt pot, plus added ingredients such as barbeque crisps, bread crusts, apple pips and anything else to hand. The challenge was to make it as big and filthy a mix as possible, and for it to be stirred clockwise with the dinner-lady's pen, otherwise it wouldn't work.
The hapless yogurt owner would then have to eat this mess. If sucessful and was able to keep the mix down for more than ten minutes they were crowned "the Great Sage" for the lunchtime. If they lost the contents of lunch within the ten minutes, they were made to eat grass, because that's what cats do when they have a dodgy stomach.