Until the stranger
Agree that it will give help only after consults to you.
Teach not to react to the phrase Mother calls.
or the father told that I took you to him.
.
Think up the password, for example, some phrase.
until the stranger told it not you him sent it.
It is possible to cross in a special way fingers, it is possible to wink or whistle.
However, this way deystven only with the senior children, kids easily give in on arrangements and can joyfully to report the password.
It is possible to agree to ring a door definitely three short, one long or to think up additional special knock.
.