(Photo Credit: Neil Crosby)
You will often see the tongue protrude gesture on display when children and adults alike are doing something that requires skill or deep concentration. Writing in the Nonverbal Dictionary of Gestures, Signs & Body Language, David Givens notes that we regularly tongue-show when performing tasks that involve precise manual dexterity e.g., threading a needle; and this may well be a reflection of the neurological link between human tool-making and speech.
Givens also notes that research at the University of Western Australia suggests that a slightly protruded tongue is employed to send out a firm "don't bother me now!" non-verbal message. So it might be best not to continue engaging in conversation with someone displaying a protruding tongue, as they may well be silently letting you know they are not interested in talking.
Finally, the non-verbal dictionary states that the "tongue-show is a universal mood sign of unspoken disagreement, disbelief, disliking, displeasure, or uncertainty. It may modify, counteract, or contradict a verbal remark. Following the statement, "Yes, I agree," e.g., a protruded tongue may suggest, "I don't agree." Tongue-shows can reveal misleading, ambiguous, or uncertain areas in dialogue, public statements, and oral testimony, and thus may signal probing points (i.e., unresolved verbal issues to be further analyzed and explored)."
Check out the protruding tongue gestures on display while this guy plays Guitar Hero III.