Where Does the Breeze Come From on a Hot Night?

One hot night, a pinwheel at the reading pavilion's door spun fast, one by the window turned slowly, and one in the center stayed still. Jibjib thought the door had its own wind; Hookhook wanted to check.

They used equal-sized pinwheels on low stands and swapped positions without changing anything else. Whichever pinwheel stood at the door spun fastest, so the pinwheel itself was not special.

Teacher Sailom closed the window while leaving the door unchanged, and the door pinwheel slowed. When the opposite window opened, air moved through and both end pinwheels turned. They used no flame or smoke.

Hookhook followed the pavilion's centerline with a short ribbon fixed to a stick. It leaned one way from window to door. Jibjib placed air-wave picture cards at each observation point.

With an adult supervising both openings, the pavilion felt cooler. Air around them was moving through the open spaces, and the pinwheels made that invisible movement visible.

We can study something invisible through its effects, and changing one factor at a time makes the answer clearer.