Numbers were invented to describe precise amounts: three teeth, seven days, twelve goats. When quantities are large, however, we do not use numbers in a precise way. We approximate using a ‘round number’ as a place mark. It is easier and more convenient. When we say, for example, that there were a hundred people at the market, we don’t mean that there were exactly one hundred people there. And when we say that the universe is 13.7 billion years old, we don’t mean exactly 13,700,000,000; we mean give or take a few hundred million years. Big numbers are understood approximately, small ones precisely, and these two systems interact uneasily. It is clear nonsense to say that next year the universe will be ‘13.7 billion and one’ years old. It will remain 13.7 billion years old for the rest of our lives.