What secret code do Liesel and Rosa concoct to communicate about Max in public?

When Max is very sick, Liesel and Rosa make a plan that if he is to wake up while Liesel is in school, Rosa will come to school and shout at Liesel for something. One day, Rosa shows up at school and shouts at Liesel for using her hairbrush. Through this interaction, Liesel knows that Max is awake.