1. Fill the large bag half full of ice, and add the rock salt. Seal the bag.Link (via Make)
2. Put milk, vanilla, and sugar into the small bag, and seal it.
3. Place the small bag inside the large one and seal again carefully.
4. Shake until mixture is ice cream, about 5 minutes.
5. Wipe off top of small bag, then open carefully and enjoy!
Update: Mapletree7 sez, "An even cooler way to make ice cream - especially on a hot day with a passsel of kids - is to put the baggie with the ice cream ingredients inside a small can, and then put that can and the ice and salt in a coffee can. Tape the lid on securely and ask the kids to kick it around the back yard."