1 Combine milk, vanilla, and sugar in the small plastic bag. Seal bag tightly, making sure to press out any air trapped inside the bag.
2 Mix ice and salt in the gallon-sized plastic bag.
3 Place the small bag into the large plastic bag and seal tightly.
4 Shake the bag vigorously for about 5 minutes. You can have fun tossing it around with friends, but you might need to wear winter gloves as the bag will be very cold.
5 After about 5 minutes, you will see that the liquid inside the smaller bag has hardened. Without opening the bags, let them sit on a plate or towel for another few minutes, rearranging the ice in the larger bag so that it's surrounding the smaller bag.
6 Open them up and scoop out the ice cream from inside the small bag. Serve immediately.