Pop quiz paradox
A teacher tells her class “there will be a pop quiz next week one day when you don’t expect it.”
One snarky student jumps up and shouts “HA! You can’t give us a quiz now!
You can’t give it on Friday because if you don’t give it Monday-Thursday then we would be expecting it on Friday.
You can’t give it on Thursday because when Wednesday night came we would know it was coming on Thursday.
You can’t give it on Wednesday because on Tuesday night we’d know it was coming on Wednesday.
You can’t give it on Tuesday because on Monday night we’d know it was going to be on Tuesday.
So we know it will be on Monday. But since we are expecting it on Monday, it can’t be then either. You can’t give us a quiz!”
"You’re right," the teacher replied defeatedly.
The next Wednesday she gave a pop quiz and the entire class was completely surprised.