If someone saw a find, an ownersless article, and fell upon it  - and another came and seized it, the one who seized it has acquired it. Formal methods of acquisition, or coming into legal possession of a movable object, include lifting it, but they do not include falling on it.

Later, Rabbis instituted another method of acquisition, where four cubits (about 6 feet) would acquire ownerless objects for the person, so that people would not come to quarrel with one another. Then why in the case above he didn't acquire through the four cubits rule? Because it works only in the public area, but he was in someone's field.