In a quiet kitchen, a kettle filled with water sits on the stove. The burner is lit, and the water inside begins to heat steadily. For a while, everything remains calm—no sounds escape, just the faint hum of the element warming the metal.
The kettle functions in this stable state, building heat without disturbance. Steam forms inside, pressure rises, but the outside world hears nothing.
Conditions reach a boundary as the water hits its boiling point. The threshold crosses precisely when steam surges through the whistle's narrow passage, setting the metal tongue vibrating.
Before this instant, the kettle stays silent. Immediately after, a shrill, continuous whistle pierces the air—the clear signal of boiled water.
The situation shifts: from quiet heating to audible alert. The kettle has crossed the line into its ready state.