Rabbi Levy is offered and accepts the role of Rabbi of Hampstead shul. Three months later, everything is going well for him except one thing - he's noticed that every time he gives a sermon, 80 year old Melvin, one of his congregants, goes to sleep. Rabbi Levy ignores this because his other congregants always say nice things to him about his sermons as they leave the shul.
But then, some weeks later, as Melvin is leaving shul, he surprises Rabbi Levy by going over to him for the first time and saying, "Thank you rabbi. I really do enjoy your sermons."
"Thank you Melvin," says Rabbi Levy, "but I've noticed for some time now that you always fall asleep during my sermons. So how can you say you enjoy them?"
"Rabbi," replies Melvin, "you should be pleased to hear that it would be impossible for me to go to sleep during your sermons if I didn't explicitly trust you."