There's a nice discussion of this in Surely You're Joking, Mr. Feynman!
> I discovered a very strange phenomenon: I could ask a question, which the students would answer immediately. But the next time I would ask the question – the same subject, and the same question, as far as I could tell – they couldn’t answer it at all!
> Then I say, “The main purpose of my talk is to demonstrate to you that no science is being taught in Brazil!”
> I discovered a very strange phenomenon: I could ask a question, which the students would answer immediately. But the next time I would ask the question – the same subject, and the same question, as far as I could tell – they couldn’t answer it at all!
> Then I say, “The main purpose of my talk is to demonstrate to you that no science is being taught in Brazil!”
https://v.cx/2010/04/feynman-brazil-education