The next time you come across a llama, whether in a zoo or in some mountain in Peru, it's probably best to not pet them. Or go very near them. Why? Because they'll spit on your face, that's why. This kid found out about this the hard way. The amusing part is that the man holding the camera seemed to have an idea that the spitting will happen. "Go on pet him on the neck." he told the boy. So pet he did. The rest is internet history.
Now, there are two main reasons why llamas spit. One, as a way of disciplining young or low ranked llamas in the herd. And two, when they are angry or irritated. The boy is not a llama. So the llama spit on his face because it got irritated by him.