How to Make Ice Cream!

What you'll need:
2 tablespoon sugar
1 cup milk or half & half
1/2 teaspoon vanilla
8 tablespoons rock salt
1 large plastic food storage bag (e.g., Ziploc) 
1 small plastic food storage bag
Ice cubes

How to make it:
Fill the large bag half full of ice, and add the rock salt. Seal the bag.
Put milk, vanilla, and sugar into the small bag, and seal it.
Place the small bag inside the large one, and seal it again carefully.
Shake until the mixture is ice cream, which takes about 5 minutes.
Wipe off the top of the small bag, then open it carefully. Enjoy!


Why does this happen?

Ice has to absorb energy in order to melt, changing the phase of water from a solid to a liquid. When you use ice to cool the ingredients for ice cream, the energy is absorbed from the ingredients and from the outside environment (like your hands, if you are holding the baggie of ice!). When you add salt to the ice, it lowers the freezing point of the ice, so even more energy has to be absorbed from the environment in order for the ice to melt. This makes the ice colder than it was before, which is how your ice cream freezes. Ideally, you would make your ice cream using 'ice cream salt', which is just salt sold as large crystals instead of the small crystals you see in table salt. The larger crystals take more time to dissolve in the water around the ice, which allows for even cooling of the ice cream.

Have a look at this video that compares a normal melting ice cube with an ice cube that has salt sprinkled on it. Which melts faster?