Water, poison, and mice
There are 3 bottles of water and 1 bottle of clear poison, but you don't know which of the 4 bottles is the poison.
You can mix (or not) the liquids into a serving for a mouse, and any amount of poison will kill a mouse. You have two mice.
What is the fewest number of feedings you need to do to ensure that you can always identify the bottle of poison?