  Understanding why the scale of emergent hadron mass is obvious in the proton
but hidden in the pion may rest on mapping the distribution functions (DFs) of
all partons within the pion and comparing them with those in the proton; and
since glue provides binding in quantum chromodynamics, the glue DF could play a
special role. Producing reliable predictions for the proton's DFs is difficult
because the proton is a three valence-body bound-state problem. As sketched
herein, the situation for the pion, a two valence-body problem, is much better,
with continuum and lattice predictions for the valence-quark and glue DFs in
agreement. This beginning of theory alignment is timely because experimental
facilities now either in operation or planning promise to realise the
longstanding goal of providing pion targets, thereby enabling precision
experimental tests of rigorous theory predictions concerning Nature's most
fundamental Nambu-Goldstone bosons.
