I'd be willing to bet that if you had at some point found yourself with an active (and at least moderately strong) desire to have a toasted marshmallow, you would have sought and found a way (oven, toaster, etc) to toast one in the kitchen... mostly because once upon a time, I found myself with a bag of marshmallows and some chocolate, and wanted s'mores, and decided since "toasting" to me at that time mostly meant "torching," a candle would suffice. And it did.
Toasting a marshmallow without a campfire wasn't a difficult problem; it was just one you didn't consciously try to solve. Maybe this marshmallow incident you've related is as simple as a recommendation for us to more actively identify little questions like that in our daily lives. If those questions can be converted to some form of desire ("I want X," or "I wish that X..."), it seems that we'd be more likely to see the simple-but-not-obvious solutions.
Over the course of working in a research lab at first part-time in undergrad, and now as a full-time grad student, I've run into a lot of little mental connections like that that tend to make me want to slap myself when I hear them, so I've started to make it a game to notice when something's unreasonably difficult, finicky, or irritating and, try to change something about it rather than just grumbling quietly to myself.
(One good example: re-papering bench space where ethidium bromide is used. Gloves are mandatory when handling EtBr-contaminated material. Labeling tape sticks to latex gloves like CRAZY, making it difficult to tape the bench paper down. It's almost embarrassing how long it took me to think of just wetting the fingertips of my gloves a bit to keep the tape from sticking to them.)
My experience with roasting marshmallows using candles is that the compounds in the wax tend to give the marshmallows a bad flavor. This may just be because of the candles I tried, but in general I had success with pushing down the toaster and roasting the marshmallow on a stick over the slots.
Today I learned that you can toast marshmallows in the oven.
By "learned", I mean "I read a recipe which included as a step toasting marshmallows in the oven". I didn't have to try it out to realize that this would obviously work. It was plain as soon as I heard the idea. And it shouldn't have needed pointing out. I know how ovens work. I am familiar with the marshmallow species of food. I love roasted marshmallows while hating them in most other forms and often occurrently lament the difficulty of arranging open flames over which one may safely toast them. I routinely try new things in the kitchen to get results I want.
And yet I read it, and was surprised. And so were the people I reported this finding to. It needed pointing out.
What other facts need pointing out, although they are plain on inspection? What is the pattern behind these facts and a good way to find more?