1. Mix all the ingredients except the salt and ice in a large mixing bowl and stir thoroughly.
2. Pour approximately 1 cup of the mixture into a small Ziploc heavy duty freezer bag.
3. Make sure the bag is secure and stays closed until the ice cream is finished or you'll end up with some salty ice cream. (Yuck!)
4. Place the small baggy into a large Ziploc bag (also heavy-duty freezer type).
5. Surround the smaller baggy with ice until the large bag is half full.
6. Pour salt into the ice, then seal the outer bag.
7. Squish and shake the bags for 5-10 minutes. The bag should become very cold to the touch, so wearing winter gloves while the ice cream freezes may be helpful.
8. When ice cream feels firm, CAREFULLY remove the inner bag from the outer bag. Serve ice cream, add any desired toppings, and enjoy!

What's going on?

This is a change in the state of matter of ice cream - it changed from a liquid to a solid. The ice cream mixture will freeze at about -4 degrees Celsius or 25 degrees Fahrenheit (water freezes at 0°C/32°F). Ice from a normal freezer is about -12°C/10°F, which is much colder than the ice cream. When the bag of ice cream mixture and the ice come in contact, some of the heat goes from the mixture to the ice, which warms up the ice but leaves the ice cream mixture colder. When you cool down a liquid, it turns into a solid, so you've got ice cream!

Actually, there's a little more going on than that. Which leads us to the question...

Why did we add salt?

A mixture of salt and water freezes at a lower temperature than pure water (this is why seawater can be much colder than freshwater without freezing.) If you live in a place where you get snow in the winter, you have probably seen this before: people put salt on roads to melt the ice.

As the salt dissolves, it melts the ice faster than ice normally would by itself. But melting ice takes energy, which it gets from the ice cream in the form of heat.

Simply put, adding salt makes the ice cream freeze faster.

Now, the ice might freeze the ice cream by itself, but because the room is so warm, the ice will probably melt before it has a chance to freeze the ice cream. To make sure that the ice cream freezes, we add the salt so the baggies get very cold, very quickly, before the air in the room has time to warm up and melt the ice.