Well, freezing, obviously. But what manner of freezing? Which is to say, if it’s so cold in that bowl, why isn’t the mix freezing into a solid block? Milk freezes solid, after all. So does cream, if left in the freezer for an extended period. So what keeps the mixture ice-cream textured? The answer is of course sugar, or more precisely, sugar syrup, which you get when the crystal sugar in the mix dissolves and combines with the water component of the milk and the cream.
What does it do? Well, as we all know, pure water freezes at 32 degrees Farenheit. Which is to say its molecules seek each other out and pile on top of one another to form big, hard crystals. Sugar molecules interfere with that process, getting between the water molecules as they try to adhere. Air bubbles and fat perform much the same service, which is what churning is all about, but more on that later.
The really, really interesting part, however is that even without all the bubbles and things getting in the way, an ice cream mix can never freeze totally. For ice, as you probably also know, can only form out of pure water. Thus, when the first water molecules in the mix manage to come together and form crystals at around 32 degrees, what they leave behind is an unfrozen sugar/water solution that’s more concentrated than it was before. Which means it has a lower freezing point. That doesn’t stop the crystallization process, at least for the time being, though as it continues an ever-more concentrated sugar syrup is created, one whose freezing point eventually drops below the capacity of the ice cream maker to freeze it. So what you have in an ice cream mix, in essence, is a solution that for all intents and purposes can never be entirely frozen. Which is an amazingly cool phenomenon, and what’s primarily responsible for keeping ice creams (and sorbets) scoop-able.