sp Hybridization in Ethyne

Ethyne has C atoms that exhibit sp hybridization.  Recall that carbon has 4 valence electrons.  In sp hybridization one s and one p orbital hybridize to form two sp hybrid orbitals.  The remaining two electrons stay in non-hybrid p orbitals.  It's these p orbitals with one electron in them on each carb that forms the two pi bonds.