Materials:
1 quart-size resealable plastic bag

1 gallon-size resealable plastic bag

1/2 c. half and half (or whole milk -- don't use skim milk as there is not enough fat to allow the ice cream to form -- you will end up with slushy, icey milk!)

1 T. sugar

ice cream flavoring (optional -- a dash of peppermint or vanilla extract works great, as does about a teaspoon of chocolate syrup)

bowl

yummy mix-ins (such as M&Ms or oreo cookies)

2 quarts ice

rock salt

Procedure:  mix together in a bowl the half and half, sugar, and flavoring.  Pour into the quart-size bag and seal.  Fill the gallon-size bag about half-full with ice and add about 2 tablespoons of rock salt. Place the quart-size bag into the bag of ice and seal the bag of ice.  Toss and shake the bag for 15-20 minutes until the ice cream forms!  To prevent your hands from getting cold, you can fold the plastic bag inside an old t-shirt.  Be careful not to break the bag!  When your ice cream is thick and creamy, mix in other yummy ingredients as desired, such as M&Ms or crushed oreos.  Enjoy!

Science:  Water freezes out of a solution as ice. In a sugar solution such as ice cream, the initial freezing point of the solution is lower than 0° C due to these dissolved sugars (freezing point depression), which is mostly a function of the sugar content of the mix. As ice crystallization begins and water freezes out, the concentration of the remaining solution of sugar is increased due to water removal and hence the freezing point is further lowered. This process of freeze concentration continues to very low temperatures. Even at the typical ice cream serving temperature of -16° C, only about 72% of the water is frozen. The rest remains as a very concentrated sugar solution. This helps to give ice cream its ability to be scooped and chewed at freezer temperatures. The air content also contributes to its scoop-ability.  Also critical to ice cream structure is ice crystal size, and the effect of recrystallization (heat shock, temperature fluctuations) on ice crystal size and texture.