People doing a tricky job will stick their tongue out and clamp it between their lips. Why? Does this happen in all cultures? 

When you need to concentrate on something, say a word problem, you are using the hemisphere of the brain also used for processing motor input. It is amusing to see people slow down when they are thinking of something difficult while walking. This is caused by interference from the two activities fighting for the same bit of brain to process them. I suppose by biting your lip or sticking your tongue out, you are suspending motor activity and also keeping your head rigid, to minimize movement, and hence interference.

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