If you’ve ever had to dig a hole and then fill it back in, you may have noticed a mysterious thing: there was always a little dirt left over. This can often be seen when construction crews dig down to get at sewer pipes or electric cables. But how can there be more dirt going in than there was coming out? The answer is that the amount of soil itself remains the same; what has changed is the arrangement of the individual grains. To see how this happens, think of a sugar bowl. If every crystal of sugar is evenly spaced, the ratio of crystals to empty space will be 60 – 40. That’s true no matter how big the bowl: 40% of the inside will always be empty.