From Uncanny X-Men #9, 1965. Looks like Cyclops isn’t the only one who needs to cool down…
Interestingly (if you’re some kind of formalist nerd) this is an example of how Silver Age comics weren’t that big on subtext. Sure, you COULD give Jean’s dialogue a double meaning and use her body language to make a point, but why bother when you can just write your subtext in a wibbly bubble?
Thought bubbles have long since fallen out of favour to the point where if someone were to use them in a mainstream superhero comic, it’d be mentioned in every review and become the subject of about fifty million blog posts about how they’re coming back (at least, this is what happened when Bendis used them in a few issues of Mighty Avengers several years ago). A shame, really, because they’re not inherently bad, just a tool in the comics arsenal, and one that’s practically unique to the form. Still, if third-person narrative captions can make a comeback, anything can.
(Also, EIGHT exclamation marks on that page. EIGHT! Most dramatic thoughts ever?)