They have to try and sound smart and call it resonance . As the positively charged nucleus spins, this moving charge creates a magnetic moment . You can think of it as a spinning subatomic bar magnet. When no magnetic field is present, these tiny magnets are aligned randomly, but when they are placed in a homogenous magnetic field, the magnetic moments will line up with the magnetic field. When the radio waves hit the spinning nuclei, they tilt even more, and even flip over sometimes.
Different nuclei resonate at different frequencies.
This means that you must hit a carbon atom with a different frequency radio wave than a hydrogen atom to get it to flip. It also means that similar atoms in different environments, such as a hydrogen attached to an oxygen and a hydrogen attached to a carbon, flip at different frequencies. By seeing at which frequencies these different nuclei flip, one can determine how a molecule is put together, as well as many other interesting properties of the molecule. That is a very good question. The answer is shielding .