Little White Rabbit Youyou's Security Password

Little White Rabbit Youyou has three sets of security codes. When someone knocks on the door, she has to change the code. When someone comes to pick her up from the kindergarten, she has to change the code. When someone talks to her, she has to change the code. How strange! Why do you need a password? Of course it's for safety! Motto: Don't trust strangers easily