On a recent trip to Target, I picked up some Slushy Magic cups that were on 70% clearance. The idea is that you freeze the ice cubes provided, add any liquid containing sugar, and put the cap on before giving the whole thing a good shake. They work fine and the kids love them. But it says right on the box that the ice cubes are made from just salt and water. It seemed so simple that I started wondering if you could make your own. I was also a little curious about how it worked.
We used the tutorial at Jill Cataldo's Super-Couponing to make our own "magic" ice cubes.
The night before you want to make a slushy, put half a cup of water and one teaspoon of salt into a plastic bag with a seal. I added some food coloring too, just for fun. Squeeze the air out of the bag and roll it into a shape that will fit inside your jar. Put the bag(s) in the freezer overnight.
When the bags have frozen put them into a jar with a lid and add a cold liquid. We used fruit and vegetable juice. Anything with sugar in it will work, but don't pour too much in. We used 6 ounces of juice for each bag.
After a few minutes, you'll see your juice becoming slushy.
Pour it into a cup and add a fun straw.
The science part (as I understand it): The salt lowers the freezing point of the water in the bag, so the bag of ice stays colder longer than plain ice. This is what causes the liquid surrounding it to begin to freeze.
You might also want to check out these other science experiments for kids: