When toast is prepared in a toaster, the toaster converts electrical energy into thermal energy. During the process of energy transformation, the law of conservation of energy states that no energy is destroyed in the process. The amount of thermal energy produced by the toaster is equal to the amount of electrical energy being supplied. The law of conservation of energy states that energy cannot be created or destroyed, only transferred into other forms.
If a pendulum is released from a height at position A, the energy that is stored as potential energy is transferred into kinetic energy. As the pendulum approaches position B the potential energy is converted into kinetic energy. While the pendulum is traveling from position B to C its energy is converted back to potential energy and the pendulum should come to rest at roughly the same height it began at position A.