Describe the law of conservation of mass. give an example.

The law of conservation of matter is the law that states matter cannot be created or destroyed, only changed in form. Here's an example of the law of conservation, but it's not mine.
"Water is stored at height with potential energy, the water flows and its potential energy is converted into kinetic energy, further it flows to rotate the turbine with mechanical energy. Then the kinetic energy is converted into electrical energy. In our house the electrical energy is converted into luminous energy, heat energy etc."

-Some examples could be these more simpler ones; Crushing a coca-cola can, cutting a block of cedar wood, boiling some water, or creating a paper airplane out of paper. ☺