The jaw is the only skull joint you move on purpose
Open your mouth and a hinge just in front of each ear lets the lower jaw drop, slide, and chew. That temporomandibular joint is the one skull joint under voluntary control.
The other skull bones meet at suture lines. In an adult those seams are locked. They are joints in the anatomical sense, but they do not open when you decide to move.
Speech, biting, and yawning all use the same hinge. The rest of the helmet stays shut.
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