The actual definition of energy leaves it very open to interpretation, energy is any force that compels a change in its environment, words with similar meanings include go, pep, zip, elan, zeal, zest, drive, force, power, and vitality. Since energy has such a broad description, it's a very hard thing to measure, when a person says he has lots of energy, it is left to the person being spoken to for interpretation.
i remember these questions from physics