Here's a puzzle that my friend Kevin sent me.*
You have five jars of pills. All the pills in one jar only are contaminated. The only way to tell which pills are contaminated is by weight. "Good" pills weigh 10 grams while "bad" (contaminated) ones weigh 9 grams. You are given a scale and can make just one measurement with it. How can you tell which jar holds the contaminated pills?
Here's the answer to a follow-up question I had; it may help you:
Q: Is the scale a balance scale?
A: No; it's a regular kitchen scale that tells you how many grams something weighs.
* Kudos to Mark for solving this in about 30 seconds.