When I took my nuclear energy course, I came up with a doubt that has remained active till today. I've search in physics books for it, but I've not found any consistent explanation. The question is why a nuclei of some element weigh less than the sum of the masses of their components?. I mean, a proton in a nucleus seems to weigh less than other out of it. If I'm wrong with that, please let me know it. Maybe I don't remember well.