Ting Peng

River water flows smoothly until it hits a narrow spot. There the current speeds up, and around a bend it may swirl into a vortex. Similar funnels, but just molecule-sized, can gather particles in one place all by themselves — no pump or motor needed.

An ordinary heat engine is like a mill between the hot and cold banks of a river: the greater the drop, the stronger the current. Here, the drop is tiny, just one or two degrees, but water can still turn the wheel. The secret is that water is made to boil not by heating but by a sudden drop in pressure, so steam bursts out even from barely warm liquid.