1. Fill a large resealble plastic bag half full of ice. Add salt. Seal and shake to mix.
2. Put milk (and sugar and vanilla, if using) in small plastic resealable bag. Seal. Check that the bag is fully sealed.
3. Place the milk bag inside the ice bag and seal again. Be sure you have sealed the bag completely!
4. Rub or shake bags for about five minutes, or until you can see that the milk has become frozen. Use mittens or oven mitts to keep hands from becoming too chilled.
5. Eat immediately!

It's worth using good quality resealable bags so that the salt doesn't seep through to your ice cream!