Fold a sheet in half and it gets twice as thick; everyone knows that part. Less obvious is what each fold costs in length: the paper has to bend around everything already folded, and the curve at the edge eats more every time, so the strip you need grows about four times over with each fold while the stack only doubles. A high school junior in California, Britney Gallivan, worked that out in December 2001, with pi in it for the curved edges, and the next month she took four thousand feet of toilet paper and folded it in half twelve times in the same direction, past the seven or eight that everyone said was the limit. I like the order she did it in. The old limit was a fact about the sheets people happened to have. She found the formula first, and then went looking for a long enough piece of paper.