A missionary has spent years teaching agriculture and "civilization" to some people in a distant land. One day, he wants to start teaching them English. So he takes the leader and points at a tree.
"Tree." He points to a rock.
"Rock." All of a sudden, they come upon two people having sex in the bushes. Embarrassed, the missionary blurts out that they are "riding a bike." Then the leader pulls out his blow gun and shoots the two people.
"What!" yells the missionary. "I've spent all this time civilizing you, and you turn around and do this!"