There's a quote I read in The Salmon of Doubt that would be perfect for this circumstance but to answer your question you can't bob your head back and forth with your mouth open because either nobody ever needed to or it was a disadvantage in the game of life and everyone with the ability to bob their head back and forth with their mouth open died.
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All that is gold does not glitter, Not all those who wander are lost
The old that is strong does not wither, Deep roots are not reached by the frost
From the ashes a fire shall be woken, A light from the shadows shall spring
Renewed shall be the blade that was broken, The crownless again shall be king