Respect.

Sticking your tongue out at someone in Tibet, is actually a sign of respect. Origins are mixed: A 9th century Tibetan king, Lang Darma, known for his cruelty, had a black tongue. Buddhists, Tibetans believe in reincarnation, and they feared that this king would be reincarnated. Tibetans have greeted one another by sticking out their tongues demonstrating that they do not have black tongues, that they are not guilty of evil deeds, that they are not incarnations of the malevolent king.

Thanks, iTraveler.