Most of the time when I ask this question at a seminar, the children say they know what a stranger is. But, when I ask the kids if they think I am a stranger, most of them respond that I am NOT. I just LOOK too nice to be a stranger. These are children ages 3-8 years old.
It can be hard to tell who a stranger is. A stranger can be young or old, even a kid that is bigger than your child. There are strangers that we see every day and sometimes, we DO talk to strangers. Our children see us chatting with store clerks, waitresses, and neighbors and it can seem hard to explain that while we are friendly to these people, they are strangers.
Helping your child to understand is really pretty easy. Have your kids name some people that come over frequently to visit, eat dinner with your family or spend the night. Not just once or twice, but many, many times.
Try to pinpoint people that are really not strangers. Some children may go to other peoples' homes to play or spend the night. Maybe you have close friends or relatives that interact with your children on a regular basis. Usually, these are the people who are not strangers. No matter how nice someone is, no matter that you see them every day, every one else is a stranger.
Point out that most strangers are just like you and your family. They are nice people that have families of their own that they love and want to keep safe the same way that you do, but, we still don't know them well enough for them to come to dinner or stay the night, so they are strangers.