Here’s a weird fact for you – a hydrogen atom has less mass than the combined mass of the proton and electron that make it up. Okay, it’s not just hydrogen – ALL elements on the periodic table weigh less than the sum of their parts.
Here’s another mind-bender: if you have two objects are made up of identical constituents, down to their very atoms, those objects will very likely have different masses. Why? That’s all down to the most famous equation in physics, Einstein’s E = mc2.