Atea kettle is warmed on an electric stove. after a while, water vapor builds inside the kettle and causes a whistle in the lid to make a sound. how does this demonstrates the first law of thermodynamics?

The first law of thermodynamics states that energy can neither be created nor destroyed, but it can be converted from one form to another form.
The electric stove is supplying heat to the kettle, this makes the water particles in the kettle to gain more kinetic energy and to increase their motion until they gather enough energy to turn to vapor. The energy gained by the water particles is used to carry out the work of whistling of the kettle.