You have been diagnosed with a life threatening disease and are prescribed two different sets of pills. You must take only one of each pill every day for a month. If you take two of the same pill, you will overdose and die.
With only two days left on your prescription, you accidentally drop the four remaining pills and they get mixed up. Unfortunately the pills look exactly the same and you have no way to tell them apart. How can you be sure you take the correct medication?