A trigger-substrate model for smiling during an automated formative quiz: engagement is the substrate, not frustration. - Harry J. Witchel; Harry L. Claxton; Daisy C. Holmes; Thomas T. Ranji; Joe D. Chalkley; Carlos P. Santos; Carina E. I. Westling; Michel F. Valstar; Matt Celuszak; Patrick Fagan (ECCE, 2018)