The law of conservation of mass states that mass is neither created nor destroyed in a closed system. A great everyday example of this is cooking.
When you prepare a meal, such as making a cake, you combine various ingredients like flour, sugar, eggs, and butter. Initially, you have a certain total mass based on the weight of these individual ingredients. After you mix them together and bake the batter, the end product—a baked cake—has the same total mass as the sum of the individual components you started with.
It might seem like the mass is changing if you notice some of the ingredients evaporating or transforming during the cooking process, like when water turns to steam. However, if you were to consider all the gases and byproducts released during baking and account for them, you would find that the total mass remains constant. This illustrates the principle perfectly and shows how mass conservation plays a role in a commonplace activity like cooking.