This puzzle is part of a famous family of puzzles and so it might be easy, if you're familiar with that set of puzzles.
You have five jars of pills. One jar of pills is contaminated. The only way to tell which pills are contaminated is by weight. A regular pill weighs 10 grams and a contaminated pill weighs 9 grams. You are given a scale and allowed to make just one measurement with it. How do you tell which jar is contaminated?