In our exploration of sound in the comics panel, we once more look at the storytelling skills of Mort Walker. This time, it's about telling us something we didn't realize about the qualities of an everyday object.
Lt. Fuzz thinks Cookie should be wearing a helmet, and Cookie reveals that he's already wearing one – in a drastic, immediately-understandable manner that will let the reader make the connection immediately. The joke hinges on the use of Cookie hitting his chef's hat with the soupçon to make an immediately recognizable metallic sound ("BONG"); if the reader would have to read Cookie's whole line explaining that "this hat is made of metal", the joke would fall entirely flat. As so often in humor, it's mostly about timing, and the visual shortcut is what makes the timing work.
(The joke is also dependent on another quality of comics: the cartoony way the hat is depicted lets Walker get away with suddenly springing the "it's metal" joke on us. In real life, it would be obvious pretty soon that Cookie's chef's hat wasn't exactly normal, but a cartoon doesn't give enough detail and realism for us to recognize that – which is why we don't react with a feeling that we've been cheated: "Hey, no fair! We would have noticed long ago if that hat was made of steel!")