1 cup Half and half (Alternatively, milk or heavy whipping cream may be used.)
1/4 teaspoon Vanilla extract
Salt (Different types of salts, such as table salt or rock salt, should work but may give slightly different results.)
Two small, sealable bags such as pint-size or sandwich-size Ziplocs
Two gallon-size sealable bags
Eight cups of ice cubes
Oven mitts or a small towel
Timer or clocks
Instructions
In each small bag, place one tablespoon of sugar, one half cup of half and half (or milk or heavy whipping cream) and one quarter teaspoon of vanilla extract. Seal up each bag after adding the ingredients. Keep the bags in the refrigerator until you are ready to continue on.
Add four cups of ice cubes to one of the large, gallon-size bags. Then add one half cup of salt to the bag.
Put one of the small bags you prepared into the large bag with the ice cubes. Be sure both bags are sealed shut.
Put on oven mitts or wrap the bag in a small towel and then shake the bag for five minutes.
OPTIONAL: Now add four cups of ice cubes to the other large, gallon-sized bag, but this time do not add any salt to it. What do you think will happen without using salt?
OPTIONAL: Put the other small bag you prepared into this large bag. Be sure both bags are sealed.
OPTIONAL: Put on oven mitts or wrap the bag in a small towel and then shake the bag for five minutes, as you did before. When five minutes are up, how do they look now compared with last time?
The Science Behind It
Freezing point depression! Because it was cold enough (several degrees below freezing), the ice cube bag with salt should have been able to cool the ingredients enough to harden them and turn them into ice cream whereas the ice cube bag without salt wasn't cold enough to do this, leaving the ingredients fluid. Although pure water freezes at 0 degrees Celsius (32 degrees Fahrenheit), water mixed with salt will only freeze at colder temperatures below 0 degrees C. This is how sea water can remain liquid at much colder temperatures than water that does not contain any salt.
Tips
The best way to ensure success when making ice cream is simply to follow the directions very carefully, especially when it comes to measuring out ingredients. Make sure to allow enough time for mixing, and be patient!