At first glance, the puzzle looks simple: four glasses labeled A, B, C, and D appear to hold nearly the same amount of water. However, each glass contains a different object — a paperclip, a baseball, an eraser, and a wristwatch. The challenge asks which glass truly contains the most water. Many people immediately judge by the visible water levels, assuming the amounts must be almost identical. That quick assumption is exactly why the puzzle has become so popular online.

The answer actually depends on a basic scientific idea called displacement. Any object placed into water takes up space inside the glass. Larger objects push away more water, leaving less room for the liquid itself. Because the paperclip in Glass A is much smaller than the other items, it displaces the least amount of space. That means Glass A contains the most actual water, even though all the glasses may appear equally full from the outside. Meanwhile, the baseball in Glass B takes up far more volume, reducing the amount of water inside.