The Igneous Petrology of Ice Cream (Thanks, Marilyn!)
Ice cream is an igneous rock. You begin with a liquid slurry containing a hodgepodge of chemicals, and by bringing it below its freezing point, you create something solid - or at least solid-ish. Good ice cream or sorbet needs a little give, a bit of liquid remaining between ice crystals so that you can comfortably dig into it with a spoon. This is what it looks like: [A scanning electron microscope image of ice cream. The ice crystals and air bubbles are separated by sugar solution From Clarke, 2003, "The Physics of Ice Cream" Physics Education 38 (3)]
Compare that to a thin section of glassy lava from the Pacific Northwest: [Small, separated mineral crystals in a glassy groundmass]
Much like igneous rocks, the same liquid mix can turn out very differently depending on what happens while it is freezing. The goal of most ice cream and sorbet is to have a smooth and creamy texture, which would be ruined by the presence of large ice crystals. To achieve this, you want to cool your ice cream so quickly that the crystals don't have time to grow, and keep the mixture stirred up while it freezes. There's a lot of energy involved in the transition from liquid to solid water, and a home ice cream maker can't do the heat transfer quickly enough to keep the ice crystals small, so you have to sit there and turn the crank until your arm is sore while the mixture slowly freezes (or invest in a fancier machine that will do the stirring for you).
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