Make Instant Ice-cream in a Bag

What can we use if we don't have rock salt?

If you can't find rock salt, use 6 tablespoons of coarse kosher salt or coarse sea salt when you pour the 6 tablespoons of rock salt onto the ice in the large bag, and if only fine table salt is available use the same amount but expect a slightly slower freeze.

My milk didn't turn into ice cream — what went wrong?

If the milk mixture hasn't thickened after 10 minutes of shaking, check that the small bag was sealed tightly with as much air pressed out, that the large bag had enough ice plus the instructed 6 tablespoons of rock salt, then shake longer or add more ice and salt.

How can I change the activity for younger or older kids?

For preschoolers, have an adult perform the step 'Ask an adult to help you pour 6 tablespoons rock salt' and do the vigorous shaking while the child helps pour 1/2 cup milk and add 1 tablespoon sugar and 1/4 teaspoon vanilla, whereas older kids can measure, seal both bags, and do the full 5–10 minute shake themselves.

How can we make the ice cream more interesting or different?

To personalize the recipe, substitute the 1/2 cup milk with 1/2 cup coconut milk or blended fruit for a different flavor, and add mix-ins like chocolate chips or chopped fruit after you 'open the small bag and scoop the ice cream' before serving.