Assume each card has a beverage on one side, and the drinker's age on the other.

Rule: if someone drinks beer, then (s)he is 21 or older.

You can see four cards (one side of each):


diet coke
23 years old
19 years old
Card #1
Card #2
Card #3
Card #4


In order to check whether the rule is true of these cards, which cards do you need to turn over?