The science behind this is actually rather complex, and is explained in much greater detail in our Instant Freeze Super-Cooled Water lab. Basically, when salt (or anything else) is dissolved in water it lowers the freezing point of the salt-water solution below that of pure water alone. In other words, when you put the salt and the ice together, you are making a solution that can get much colder than 32 degrees Fahrenheit (the normal freezing or melting temperature of pure water) and still remain liquid. This is called freezing point depression, and it can be very useful when you are trying to get something else really cold (in this case, the ice cream). Freezing point depression is also why the milk or cream needs to be much colder than 32°F before it will freeze; milk is mostly made of water, but with a lot of other stuff dissolved in it, and that lowers its freezing point too. As you shake the big bag, sloshing around the smaller bag inside, the super cold salt-ice-water solution in the big bag takes heat away from the cream/milk inside the smaller bag, lowering the temperature enough for it to freeze. You should also notice that this heat flowing out of the small bag melts even more of the ice in the big bag- it gets sloshier as you shake it. Of course heat from your hands does the same thing (and you might want to wear gloves as you shake the bags).
It's very important that you shake the bags vigorously while the milk/cream is freezing. This breaks up the ice crystals that are forming inside the smaller bag and keeps your final ice cream smooth and creamy- just the way you probably like it. If you don't shake the bags, as these ice crystals form they will stick to each other and you'll end up with a hard frozen block of milk instead- a milk-sicle! Try it by placing another Zip-Lock bag or cup of milk in your freezer to see what happens.
Variations and Related Activities:
Here's another way to make ice cream, which uses the same ideas, but slightly different materials:
You can play around with many different ways to make ice cream. What if you used skim milk? 2% milk? A combination of the two? Does it take longer for them to freeze? What if you used chocolate milk? Do you prefer richer ice cream (made with pure cream), or not-so-rich ice cream (made with Half and Half)? What about ice milk (which is ice cream made with milk and no cream at all)? Which tastes the best?
Another way to do this is to get two metal cans (coffee cans work well). One should be able to fit inside the other, with room for the ice and salt. It should also have a very tight-fitting lid. Put the milk/cream mix inside the small can, and put the lid on it. You might want to use duct tape to make sure that the lid stays on. Then, put the small can inside the big can. Put the ice, salt and a little liquid water inside the big can, around the small can. Put the lid on the big can, and duct tape it shut. Then, play soccer with the can for about 15-20 minutes! Untape the cans, wipe off the little can, and enjoy the ice cream!
Links to more information and activities:
More versions of this activity:
The science of ice cream:
A gourmet recipe for ice cream in a bag:
Turn this activity into a real experiment: