At first glance, all four glasses seem to hold the same amount of water. But the trick lies in displacement — the objects inside each glass take up space. The larger the object, the less room remains for water.
Since the paperclip in Glass A is the smallest object, it displaces the least water. That means Glass A actually contains the most water, even if the water levels look similar.
This puzzle became popular because it shows how easily people rely on first impressions instead of noticing small but important details. What seems obvious at first can completely change once you look more carefully.