[Potironette]

Woah! So energy is mass..? Or most of mass is energy? Doesn't that mean mass is literally energy bunched up together O_o? And that what's happening at nuclear power plants is more or less spreading that locked and bunched together energy out?

Nope! I mentioned this before: The energy absorbed or released by chemical and nuclear reactions does in fact influence the system's rest mass! And we can use E=mc² to describe just how much.

Ohh woops. When energy is released mass is lost, and taking the stuff apart means releasing all the energy. But then again protons and electrons aren't even masses either (I forgot quarks existed, well I never knew what they were until now ^^;;)...What even is mass, it seems that literally everything might as well be internal forces o_o.

Oh! But why do quarks count as rest mass? Aren't they energy as well x'D.

So really, what I was saying that we don't know is if there's a contribution to the total E of a system that you can't measure by observing its rest mass or its momentum.

The internal forces aren't accounted for is because the internal forces are part of the rest mass..? And they, because of Newton's third law, aren't going to suddenly change the object's rest mass?