1 Why Some Surprises are More Surprising than Others: Surprise as a Metacognitive Sense of Explanatory Difficulty Meadhbh I. Foster 1 and Mark T. Keane 1 1 School of Computer Science and Informatics, University College Dublin, Ireland This manuscript is the preprint of a paper that will be published in Cognitive Psychology doi: 10.1016/j.cogpsych.2015.08.004. © 2015. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/ Running head: EXPLAINING SURPRISE Correspondence concerning this article should be addressed to: Meadhbh I. Foster School of Computer Science and Informatics University College Dublin Belfield, Dublin 4 Ireland E-mail: meadhbh.foster@ucdconnect.ie Telephone: 00353 (0)1 7162907