atomic nuclei are able to sense temperature because of the thermal energy that is present in the environment. This thermal energy is composed of the kinetic energy of the particles in the environment, which is determined by the temperature. As the temperature increases, the kinetic energy of the particles increases, and the protons can move around more, allowing them to absorb or release energy. This energy can be used to change the spin of the protons, which is what happens in the case of ortho and para hydrogen.