In a time before texting, email and instant calls, parents would create a passphrase that only they and their children knew. For example, “I’m feeling hungry. Banana sandwiches are the best”. If some guy shows up to their school and says “I’m here to pick you up from school, Carrie – your mom – sent me, something’s wrong at home. I don’t have details but you need to come with me, quick.” The kid says, “Okay, I’m feeling hungry.” If the guy doesn’t come back with “Banana sandwiches are the best.”, the kid needs to run like hell.
On the other hand if he does – as instructed by the parent – the child knows he or she can trust him.
(For Battlestar Galactica fans, another phrase you can use – “Whaddya hear?” “Nothin’ but the rain”.)
This now forgotten system is becoming necessary again as, given a sufficient sample, computers can make anyone say anything. How else would you really know it’s your loved one on the other end of the phone line?