You fill a pot with water and set it on the stove. The burner glows beneath, and the water begins to warm. Small bubbles form along the bottom, clinging to the metal before slowly rising. The surface stays mostly smooth, with only faint ripples now and then.
The bubbles grow slightly larger and more frequent at the base. They release quietly underwater, barely touching the top.
Then comes the shift. Large bubbles surge upward from the depths and burst on the surface with a pop. More follow in quick succession.
Before, the water lies calm across the top. After, the surface churns constantly with rising and breaking bubbles. Steam rises steadily from every point. The pot fills with motion and sound where stillness held.
The water has crossed into boiling.