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.