Two bottle of pills must be taken to keep you alive. The two must be taken at the same time. Forgetting a pill, taking one and not the other, or taking two of the same one will result in your death. You open the two bottles and pour them both into your hand without thinking. Three pills fall out on your hand. Unfortunately the two pills look, feel, and weigh exactly the same. How do you ensure that you don't take the wrong pill?
Assume that these pills are super expensive and cannot be discarded. So throwing the pills away and just picking one pill each is not a viable option.