Here's another great recent Savage Chickens comic by Doug Savage. I've commented on this great strip before, and here's another good "meta"-comic:
As we all know, hearts are symbolic of love (especially over the head), while lightbulbs over the head represent inspiration. I call "over the head" symbols like these "upfixes" since they are like affixes that are "up" (a term coined by my mentor, Ray Jackendoff).
Of course, as in the last panel, a heart and a lightbulb together show a love of lightbulbs, not a love of inspiration or ideas. This combination also does not give you inspiration about love either. What's interesting about Savage's observation here is that it nicely shows that you cannot combine the upfixes together. This is a first interesting trait: these upfixes have meaning on their own, but not in combination.
Related to this, the nature of the lightbulb upfix is to give you "value added" for its meaning. It no longer is just a lightbulb, but out of the relation above the head generates a new conventionalized meaning of inspiration. By adding the heart, it effectively removes this additive meaning, making the lightbulb simply a lightbulb once again.
The heart also changes meaning a bit as well. When the heart is an upfix, it describes the mental state of the person: The chicken is in love—with what, it doesn't matter. However, in the third panel, their combination makes the heart modify the lightbulb now—it's a love of lightbulbs—not merely reflecting a general mental state of the lover (the chicken).
Finally, this combination also changes the thought bubble. In the first two upfixes, the thought bubble mostly gives a depicted link between head and upfix. It doesn't mean thinking about love or about inspiration, but just reinforces these signs as being mental states. However, it is mostly unnecessary. The heart or the lightbulb would retain their meaning without the thought bubble. In the combination though, the bubble now returns to it's usual meaning as encapsulating thoughts. Having a heart and a lightbulb floating above the head wouldn't work as an upfix, nor would it work to convey thoughts.
Altogether, this simple, quirky comic tells us a lot about the structure of these types of signs!