The Growing Round
Matthew, Mark and Luke are at the pub. John shows up to join them. What does he do?
He must first assess the situation by taking note of the level of the lowest drink. If that drink is more than half empty (always half empty, never half full) AND the owner of that drink would like another, then John must buy a full round. Otherwise John may buy a drink for himself only. In the latter case, it will be John's round after Matthew, Mark and Luke have bought a round.
Now let's imagine that John has bought a drink for himself only, and the round has progressed through Matthew and Mark, when Lisa arrives (Note: Lisa is a woman; see Women in the Round to find out why this is worth mentioning). Lisa assesses the situation and sees that Matthew has almost finished his drink. She rightly asks if he would like another, and when he responds in the affirmative, she trots off and buys a full round; the next round will return to Luke.
Note that when each new member of our Group Of Drinkers arrives and buys a full round, the other members' obligation for the round will again be deferred, but when their time does come, their rounds will be larger. This is what's referred to as The Balance of the Universe.
The Growing Round can become more complex when a group of Sober Individuals arrive. Let's go back to Matthew, Mark and Luke in a round. John, Lisa and Eve arrive (see Who's First for details). Lisa sees that Mark's glass is almost empty and so buys a round for all. In turn, John and Eve must also 'buy into' the round before the round resumes with Matthew, Mark and Luke. If John, Lisa and Eve had already been in a round then the Mixed Rounds rules apply.