Alveoli

seventy square metres, folded into six litres.

fall through your lung — every split doubles the surface hidden in your chest

Each branch splits in two, and exposed surface roughly doubles with every split: a plain rule repeated, not one jump. Real lungs carry it to twenty-three generations, the trick the gut and kidneys use too.

Concepts: anatomy, scaling.

An interactive you play with one thumb, on Wundr.