Because the jaw has shifted.
In order to put the head of the light bulb into the mouth, it is necessary to open the mouth as far as possible. At this time, a part of the jaw joint (the scientific name of the joint disc) moves forward together with the mandible. When you put your hand on the cheek, you can feel the jaw joint pushing outward, and the muscles relax at this time. , the ligaments can be stretched further, and the mouth can be opened at a greater angle. Wait until the head of the bulb is inserted into the mouth and the smooth bulb begins to slide inward. At this time, because the neck of the bulb is relatively thin, the pressure on the oral ligament is relatively small, and the pressure near the base of the tongue is relatively large, so the joint disc that moves forward is reset backward, and the bulb is stuck in the mouth. But at this time, the articular disc is supported by the bulb head and cannot move forward again, and it is very difficult to open the mouth wide.