The mind can only comprehend...what the buttocks can endure! This is a saying about long classes and seminars. If attendees have sat to the point of discomfort, they probably won't pay attention to the class.
The students had glazed eyes and shifted in their seats. The mind can only comprehend...what the buttocks can endure!
means when you are talking to someone with attitude and they don't understand you, then you have to redirect them to full comprehension of the conversation.