The Bad Son
In the episode where Frasier falls the Retirement Home manager, he takes Marty along to the home as a ruse. Miranda offers to take them on a 20 minute tour and Marty (who's anxious to get to the game) says, "A tour?! Twenty minutes?!"
Frasier starts stroking his head and says soothingly, "Yes dad, a tour. Twenty minutes."
Watch Marty at this point (in fact it happens twice). He's clearly smiling. Given that Kelsey Grammar often did little preparation and preferred to go in cold to a scene, do you think he ad-libbed this and John Mahoney struggled to stifle a smile? His face is one of genuine amusement, rather than irritation at being patronised like a senile old man!