how do you know this?

As we were driving home from church, I was checking my twitter on my phone. One tweet caught my eye. It was a mom that mentioned her dog and a gross thing that dogs do. I read it aloud to my family. We got a chuckle. That got us to talking about the nasty things our dogs are caught doing. Then Jonah, speaking like he knew what he was talking about, said,

"Oh yeah, like when they lick my armpit!"