A stranger is a person who is unknown to another person or group. Because of this unknown status, a stranger may be perceived as a threat until their identity and character can be ascertained. Different classes of strangers have been identified for social science purposes, and the tendency for strangers and foreigners to overlap has been examined. The presence of a stranger can throw an established social order into question, "because the stranger is neither friend nor enemy; and because he may be both". The distrust of strangers has led to the concept of stranger danger (and the expression "don't talk to strangers"), wherein excessive emphasis is given to teaching children to fear strangers despite the most common sources of abduction or abuse being people known to the child.