Water and Vinegar
The old puzzle was you have one litre of water in one bucket and one litre of vinegar in another bucket. Take a spoonful of water from the first bucket and mix it thoroughly with the vinegar in the second. Now take a spoonful of the vinegar-water mixture in the second bucket and mix it back with the water in the first. Problem: does the first bucket have more vinegar or the second bucket more water? Solve it only if you can get at the most elegant solution with no math involved at all.