Consider a typical haystack. Now, instead of the traditional challenge of trying to find a needle, which is a distinctly different material and shape from the hay, imagine instead trying to find a single rotten piece of hay. That rotten piece is softer and browner than normal hay—and that’s all that sets it apart. How would you even begin? Now, imagine that the one rotten piece of hay can start to turn nearby pieces of hay rotten as well, and the longer it takes you to find the rotten hay pieces, the more pieces of hay start rotting.