... and if that's too grand a scale for you, so do the air pipes in your lungs:
Dr. Andre Kempe/Oxford Scientific/Getty Images
Smokers' lungs look like the Fibonacci sequence.
This branching is due to fractal systems known as dendritic configurations. Every system in which something flows -- be it water, blood, or electricity -- tends to branch and bifurcate in a manner seen above. Parent vessels in these systems, like arteries, air pipes, lung capillaries, and various transport systems in plants, branch into smaller daughter vessels to distribute the nutrients or blood (or whatever the vessels are carrying) as effectively as possible. Trees mostly follow the bifurcation system where one divides into two. The tree trunk divides into several branches so that the leaves are spread on a largest area possible to receive sunlight. Rivers form deltas for similar reasons. When a river starts emptying out into an ocean or a lake, it slows down, and all the silt starts to settle, so bifurcating is a better option than trying to hose it through like a thumb on a garden hose.
It's not as dirty as it sounds.