Why can't I move my.....teeth?

While standing on the stool in front of the bathroom mirror, Maggie said, "Bill can move his teeth in and out with his tongue, kind of like a box.  Like this."  She made a motion with her tongue.  "I'm trying, but I can't move mine."  Laughing, I told her I can't move mine either.  "Why", she asked.  :)  She found it very interesting when I explained to her how some people take theirs out to clean them.