When Moby and Sarah Byrnes go for a walk, they are watched by her father. How do they communicate since Sarah Byrnes can't be seen talking?

When Moby and Sarah Byrnes go for a walk and are watched by her father, Sarah Byrnes and Moby communicate by hand squeezes. Moby asks Sarah Byrnes questions and she answers by squeezing his hand one time for yes and two times for no.