When I think of marshmallows, I imagine fluffy clouds of sugar, so I'm always a little disappointed when I open a box of cereal and see crunchy, cereal marshmallows. Sure, they're delicious, but it begs the question: Why are cereal marshmallows hard? After all, in any other context, a crunchy marshmallow is something to be avoided, a sign of staleness. Cereal marshmallows are crunchy because they are dehydrated, which means that most of the water content has been removed. That's what gives cereal marshmallows their crispy and crumbly texture.
But there might also be a practical reason that cereal manufacturers use dehydrated marshmallows rather than the standard, fluffy marshmallows you'd roast over a fire. YouTuber Cereal Snob, a.k.a. Thomas Hicks, runs an entire channel devoted to reviews of different cereals. He was wondering why marshmallows in cereal were hard and if they would "be good if they were squishy anyway?" His hypothesis was simple. If the marshmallows are soft, that means they're full of water, and if you combine those soft marshmallows with the crunch, crispy corn cereal, chances are good the marshmallows would turn the whole box stale and squishy.
To find out what happens when you replace the dehydrated marshmallows that come in a standard box of cereal with standard, non-dehydrated miniature marshmallows, Hicks conducted a science experiment. He took two cereals with marshmallows—Lucky Charms and Smorz—and removed the dehydrated marshmallows from a sample of each one. Then he replaced them with regular, soft mini marshmallows and put the mixture in a plastic baggie. In another set of plastic baggies, he put the regular cereal with the dehydrated marshmallows as a control. After 13 days, Hicks tasted each of the cereals to find out what happened.
Long story short, the cereal with the soft marshmallows tasted stale while the cereal with the dehydrated marshmallows still had some crunch to it. Plus, the squishy marshmallows were harder and "a little sadder than they used to be," which seems like a lose-lose all around. If you do want fluffy marshmallows on your cereal, however, just put a handful on top before eating. That way, you don't ruin the whole box.