Largest boulder size that can be lifted by outgassing as a function of the comet activity expressed as dm/ dtv_g, where v_g is the gas velocity at the surface and dm/ dt is the local gas-loss rate. The purple line shows the largest diameter for the highest surface gravity that can be encountered on the surface of 67P, while the red line outlines the largest boulder size when the lowest gravity of 67P is considered. The outgassing values for comet 46P Wirtanen and 1P Halley are presented together with the range (gray) of dm/ dtv_g for 67P following Bertaux et al. (2014).