How Do They Know?

How does a dog know where you hurt? So many days when my knee is bothering me one or more of my dogs will come up when I am sitting and gently lick right on the sore spot. It's not as if there's a sign there. But they are uncanny at knowing exactly where to give aid. Maybe the sore spot is hotter than surrounding tissue. Or maybe there is some kind of throbbing that I can't feel underneath the normal pain that they can sense. Or maybe they just are God's angels. Amazing......