When a flounder hatches from an egg, it has two eyes symmetrically placed on either side of its head, just like any other small fish. However, when it grows for about 20 days, the body grows to 1 centimeter long, a strange thing happened. The eye on one side of the flounder began to move. It passes through the head's . The upper edge of the head gradually moves to the opposite side, and it doesn't stop moving until it gets close to the other eye. Different species of flounder have different methods and routes for moving their eyes. The skull of the flounder is made of cartilage. When the flounder's eyes start to move, the cartilage between the flounder's eyes is first absorbed by the body. In this way, there is no obstacle to the movement of the eyes. The flounder's body structure and organs also change when the flounder's eyes move. The halibut hunts by hiding in the sand and waiting for a smaller fish to swim by and wait for an opportunity to kill it. Having both eyes on the same side prevents sand from getting into the eyes and damaging them when they are lying on their side on the sea bed.