Why does an electric kettle whistle when it's first turned on?

An electric kettle whistles when it's first turned on because of the steam generated as the water heats up. As the water reaches its boiling point, steam escapes through a small opening in the lid or spout, creating a pressure difference. This pressure forces the steam to vibrate, producing the whistling sound. The whistle serves as an audible signal that the water has reached its boiling point.