A whistling kettle will start to whistle when the water boils and turns into a jet of steam which then exits the small aperture in the spout.
But why doesn't this happen much earlier - when the air molecules in the kettle get heated up enough, shouldn't they also (due to the increased pressure) forcefully escape through the aperture? (Assumee that the kettle was half filled with water, and so the upper half was filled with air). This should happen long before boiling (which is when the water molecules gain enough momentum to escape the water surface). Yet in practice, the whistling only starts at the time the water boils - how come?