Leaves. Why do they do that? London pavements are covered in fallen brown leaves right now, but only in places. They form huge piles, all huddled together in clusters. Why do they do that? Why do they choose just that spot along the wall? Why are there whole empty stretches with not a single stray leaf, and then occcasional, seemingly random, leafy mounds? I'm pretty sure it's not that someone has swept them up into these heaps - surely they would have carted them away too. Strangely, within each pile there are gaps, empty spaces where no leaf dare go. Why do they do that?