A statistic which tore through the school at an alarming rate, representing the average volume of saliva present in the final dregs of a drink. It was the unusual precision of the number which led to its universal acceptance. A natural logical leap stipulated that any drink which was under half full was up to 97% owner spittle. Only the first sip was safe if you didn't want to French Kiss all the previous sippers.
If you wanted to upgrade the statistic yourself, you could do so by saying "they've just found out..." before giving your new figures.