The energy which is contained in a body is called internal. To learn its quantity, it is enough to increase a body weight square by the speed of light. In practice, however, it is impossible to extract all this energy - it is possible to get only its rather small part.
1. In combustible substances the internal energy contains in a chemical form. For its extraction burn a body in the atmosphere of air, at the same time transforming the turned-out heat to mechanical or electrical energy by means of the steam-engine, the Stirling engine, the thermocouple, etc. Some bodies burn only in the atmosphere of pure oxygen, chlorine or other gases, and with air refuse to react even when heating. You make experiments of this sort only in the presence of the teacher of physics or chemistry.
2. In a battery a form of storage of internal energy also is chemical. Plates or cylinders from the metals which are easily reacting, for example, zinc or lithium act as "storages" here. Until the element is connected to anything, its self-discharge is insignificant. Connect to it loading, for example, the bulb or the electric motor suitable in parameters, and process of extraction of the reacting electrode of internal energy and its conversion will begin at first in electric, and then - in light or mechanical. Do not close an element short.
3. If the body is located at some height to the Earth's surface, it has potential energy which depends on the weight and height. That to extract this energy quickly, having turned into kinetic, reset a body down. It is possible to extract it and slowly, having suspended a body on a chain of the pendulum clock as the weight. Cargo will fall with a small speed, gradually giving the energy to the mechanism of hours.
4. Buy the toy or a sticker glowing in the dark. Bring it to a light source - and the phosphor which is contained in it "will be loaded" with energy of electromagnetic waves. Now to extract energy from a phosphor, just turn off the light. The phosphor will begin to radiate light itself, but with much smaller brightness.
5. Have the greatest power consumption nuclear, and especially - thermonuclear sources. They allow to extract huge amount of energy which cannot be got from it in other ways from a body. Do not try to make similar experiments independently - they are extremely dangerous.