Supporting considerations in the paper: the X_max distribution shows the composition getting lighter through the dip region; the picture is consistent with the absence of statistically significant anisotropy; the break near 10^17.5 eV may mark the energy of maximum heaviness. For the main question, the composition (proton vs iron fraction) at ~10^17-10^20 eV controls how many nucleon-nucleon collisions above LHC CM energy nature provides, since only 1/A of a nucleus’ energy enters each nucleon-nucleon collision.