The GZK limit exists as a limit on the highest possible cosmic-ray energy that can travel without interaction through the universe, and cosmic rays above around 5 x 10 eV can reach Earth only from the nearby universe. The limit exists because at these higher energies, and at travel distances further than 50 Mpc, interactions of cosmic rays with the CMB photons increase. With these interactions, the new cosmic-ray product particles have lower and lower energy, and cosmic rays above a few 10 eV do not reach Earth (except if their source would be very close). Important in this context is that the GZK interactions also produce neutrinos, called cosmogenic neutrinos. Their energy is typically one order of magnitude below the energy per nucleon of the cosmic ray particle (e.g., a 10 eV proton would lead to 10 eV neutrinos, but a 10 eV iron nucleus with 56 nucleons, would lead to neutrions of 56 times lower energy than for the proton case).