[I read this problem in a puzzle book I have.]
You have two jars. One contains vinegar, the other oil. The two jars contain the same amount of their respective fluid.
Take a spoonful of the vinegar and transfer it to the jar of oil. Then, take a spoonful of liquid from the oil jar and transfer it to the vinegar jar. Stir. Now, how does the dilution of vinegar in the vinegar jar compare to the dilution of oil in the oil jar?