This fun charm is wonderfully detailed for such a small piece: The eyes, nose and mouth are all cut out so that you can see into the cranium, and there's a full set of defined individual teeth. The view from the side looks realistic too, the way the jaw appears to jut forward because there's no nose.
And of course it's hollow like a real skull, and there's a good use of antique effect to highlight all the details and to make the inside cavity look deep.