It is closer to how you stated it originally -- dropping a grain of sand into your coffee cup could
cause it to boil. Occasionally, this happens -- but it's usually not sand. It may be sugar or something. If you heat water on the stove in a very smooth pot without disturbing it, it can reach a superheated state, at which the slightest agitation causes it to immediately "explode" out of the pot and cause burns. Check this link out for more info:
http://www.snopes.com/science/microwave.asp
Also, I think you are a confusing a little bit the concept of energy and Gibbs free energy. Gibbs free energy is the energy available for doing work. For some systems, you can "extract" energy into a different form. But, for example, a heat death universe would be full of energy -- yet you can accomplish no work. So what is energy then? Lubos Motl (theoretical physicist), gives a much better explanation than I can:
http://physics.stackexchange.com/que...d-it-come-from. Feynman also has a very good explanation.
ALL ENERGY is affected by gravity, since it IS mass. That's my way of describing it. A compressed spring weighs more than an uncompressed spring. A spinning ball weighs more than a stationary one.
I'm not sure why you say there are fundamental units and secondary units. Most physicists consider energy perhaps the most fundamental quantity in the universe, of which everything else can be derived. Heck, string theory postulates that everything is composed of 1D "strings" that resonate at different energy frequencies. Why did the Hamiltonian get famous over the momentum, position, or spin operators? It's because it was energy related.
If I missed something you wished address, or I confused you somehow, let me know.
(Yeah, I know I sidetracked stuff a bit -- to others, don't read it then).