Look Mom, I'm spitting

At what age do boys start this whole spitting ickiness?

Here's my son, getting ready for baseball practice with a wad of sunflower seeds stored in his cheek. I asked him to pose for me and as I shot this one, I caught the spit flying out of his mouth.

And he was proud. And he wanted me to take more pictures of him spitting. And I did.

But, I must ask again. When do boys start spitting? He certainly didn't learn it from me.