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Is there a word that refers to that particular facial expression? I am trying to think of something, but can't think of anything, but I think I heard someone use it before. You see cartoon characters in Bugs Bunny and other American cartoons for children make that expression.

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A word that come to mind is "agog". This is an adjective or adverb. It mostly refers to eyes wide open, but generally conveys the idea of "wide-eyed and open-mouthed in astonishment". Relevant usage examples from https://www.lexico.com/en/definition/agog are:

‘Gilz and I sat open mouthed and agog for most of it.’
‘Matt stood with his mouth open, agog at what he had just heard.’

Comments  

The only thing I can think of is "double take."

 GPY's reply was promoted to an answer.