In most countries sticking out your tongue would be considered a bit rude, but in Tibet it's a way of greeting.
It has been a tradition since the 9th century, the time of an unpopular king called Lang Darma, who was known for his black tongue.
People in Tibet, thought that the king had been reborn, so to prove they weren't the king, they would show their tongues.
The traditional greeting is now a form of respect.