This is a quite old puzzle, although this version was inspired by a recent post on Richard Wiseman's Blog.
You have an old soda vending machine used in blind taste tests. The machine has three buttons labeled, "Coke," "Pepsi," and "Surprise." Whichever button is pressed, a nondescript can of soda pops out containing either Pepsi or Coke - the surprise button is supposed to randomly give a can of one or the other. Trouble is, some rascal has gone and switched the labels so now every label is wrong. Luckily, you are an expert at telling the difference between Pepsi and Coke, so if you sample one of the cans, you will be able to determine which soda it is (even though the can is no help). How many sodas must you drink to guarantee you will be able to put the labels back where they belong?
Richard Wiseman's Blog - new puzzle every Friday, answers on Monday.