Materials
1/2 cup of milk
1/2 teaspoon vanilla
1 tablespoon sugar
4 cups crushed ice
Rock salt
2 quart size Zip-loc bags
1 gallon size Zip-loc freezer bag

Procedure

• Mix the milk, vanilla, and sugar together in one of the quart size bags.
• Seal tightly, allowing as little air to remain in the bag as possible. Too much air left inside may force the bag open during shaking.
• Place this bag inside the other quart size bag, again leaving as little air inside as possible and sealing well. By double-bagging, the risk of salt and ice leaking into the ice cream is minimized.
• Put the two bags inside the gallon size bag and fill the bag with ice, then sprinkle salt on top. Again let all the air escape and seal the bag.
• Wrap the bag in the towel or put your gloves on, and shake and massage the bag, making sure the ice surrounds the cream mixture. Five to eight minutes is adequate time for the mixture to freeze into ice cream.

What happened?
Salt mixes with ice, causing the ice to melt. When salt comes into contact with ice, the freezing point of the ice is lowered, meaning it takes more energy to melt the ice. Water will normally freeze at 32 degrees F. This makes the ice colder than it was before, which is how your ice cream freezes. Yum!

What does the salt do?
Just like we use salt on icy roads in the winter, salt mixed with ice in this case also causes the ice to melt. When salt comes into contact with ice, the freezing point of the ice is lowered. Water will normally freeze at 32 degrees F. A 10% salt solution freezes at 20 degrees F, and a 20% solution freezes at 2 degrees F. By lowering the temperature at which ice is frozen, we are able to create an environment in which the milk mixture can freeze at a temperature below 32 degrees F into ice cream.