The Little Toe
It appears that Luo and the seamstress have one thing in common, their second toes are longer than the others. This seems to be a very uncommon trait, yet these two people have it. A coincidence like this would appear to be a symbol of how Luo and the seamstress might be destined to be with one another. On the other hand, it could also be a symbol of modern science, like charges don't attract.
