Additional supplies needed: A 1 lb coffee can and a 3 lb coffee can.
Mix the first three basic ingredients together (not including rock salt) and pour into the one pound coffee can. Add flavorings if desired. Place the lid on securely and set inside the three pound can.
Add ice, and alternate layers of ice and salt outside the small can and inside the large can. When totally full, secure the lid on the large can. You may wish to add duct tape for good measure.
Now here comes the fun part! Place a sheet on the floor or wrap the can in a large towel. Let party guests roll the can back and forth for at least 10 minutes. Open the cans and check the ice cream.
If it is not starting to freeze, replace lids and roll 10 minutes more. The ice cream should be frozen to the sides and bottom of the can. If not thick enough, place in freezer and check every 10 minutes or so.
Note: This recipe can be effected by temperature, humidity, and other environmental factors, so make a trial run before attempting in a group setting.