('informal') You need to 'get it into/through your thick skull' [=you need to realize/understand] that you can't just sit there and expect everyone else to do your work for you!
('informal') He can't seem to 'get it through his skull' that I don't like him.
He had a fractured 'skull'.
(..) of a projectile... ...as it hits a target and the destruction that it causes. Take, for instance, what you carry, mm hollow-point. When the hollow-point hits the skull, what happens is it mushrooms. When it mushrooms it peels back. So you may have six, eight... ...ten little particles (..)2006 The Departed