system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is the atomic mass unit definedas "one-twelfth of the mass of one atom of carbon-12"? Why specifically carbon and not just the mass of one proton?
gpt
Because the "mass of a proton" is not as easy to define as you think.

It turns out that if you take apart a carbon atom or really any atom into a bunch of protons and neutrons, the protons and neutrons will have more mass than the carbon atom itself. This is because free protons/neutrons have more energy than a carbon atom, and by Einstein's E=mc^2 they thus have more mass.

In fact there used to be an older definition of the AMU that used oxygen instead. But oxygen has slightly different binding energy to carbon and as such a slightly different amu, the scientific community came to a consensus that both definitions could not be used. 

Regardless though we could have defined the AMU to be the mass of a free proton/neutron. The reason seems to be because carbon is a convenient element. Its solid at room temperature and pressure, pretty chemically stable, and as such experiments could be conducted on things made from carbon pretty easily to determine the count of the carbon atoms in it.