Calculate axb

A Priest was about to finish his tour of duty in the jungle where he has spent years teaching the natives to be civilized and kind to each other when he realizes that the one thing he never taught them was how to speak English.
So he takes the chief for a walk in the forest. He points to a tree and says to the chief, "This is a tree."
The chief looks at the tree and grunts, "Tree."
The Priest is pleased with the response. They walk a little further and he points to a rock and says, "This is a rock."
Hearing this, the chief looks and grunts, "Rock."
The Priest was really getting enthusiastic about the results when he hears a rustling in the bushes. As they peek over the top, he sees a couple of natives in the midst of heavy sexual activity.
The Priest is really flustered and quickly responds, "Man riding a bike."
The chief looks at the couple briefly, pulls out his blowgun and kills them both..
The Priest goes ballistic and yells at the chief that he has spent years teaching the tribe how to be civilized and be kind to each other, so how could he kill these people in cold blood that way?
The chief replied, "My bike."