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 the change in the internal energy of a system equals the net heat transfer into the system minus the net work done by the system. This is another way of saying that energy can neither be created nor destroyed, but it can be converted from one form to another form. 
A tea kettle that is warmed on an electric stove is receiving heat energy, this heat energy increases the kinetic energy of the water particles in the kettle and makes them to move faster. As time goes on, the water begins to turn into vapors. The heat energy that is released into the system is been used to carry out the work of evaporation and the whistling of the kettle.

The example above shows the first law of thermodynamics because it shows that heat cannot be created or destroyed but only transfers from one source to another, and be transferred from one location to another. This example shows this because the heat of the stove is transferred to the tea kettle and some of the heat dispersed to the air. The heat from the kettle transfers to the water which causes the water to boil, then the boiling water turns to steam which fills the space inside of the kettle until as much heat is held in that the kettle can hold, then the excess steam escapes cause the tea kettle to whistle.

Centrapedial force is the answer