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Here's a quick puzzle to get your mental mechanics going.
You are shown four cards. You know that each card definitely has a letter on one side and a number on the other.
However, somebody you don't trust also tells you that if any of these cards has a D on one side, it also has a 3 on the other.
The four cards show: F 3 D 7
What is the least number of cards you would need to turn over to find out whether you were being told the truth, and which ones are they?
Think carefully - don't rush your answer! Once you've worked it out, there's an interactive tester below to check your solution.
If you're interested in the psychology behind this puzzle, you might like to check out Steven Pinker's recent book How The Mind Works for much more information on the psychology of puzzles like this.
Which cards do you want to turn?