This problem is a short, sharp lesson in what it means to check a rule. The winner is not whoever turns the most cards, but whoever rigorously distinguishes which ones could refute it.
The four cards
Riddle statement
Envelope the mesa there are four cards. Each one has a letter on one side and a number on the other.
You see these faces:
A D 4 7
You want to check if this rule is true:
“If a card has a vowel on one side, then it has an even number on the other.”
Which cards do you need to turn over, and only those?