Make Ice Cream with Science

What can we use instead of rock salt or half-and-half if we can't find them?

You can replace rock salt with coarse kosher or regular table salt (use a bit more if it's fine-grained) and swap half-and-half with heavy cream or whole milk using the same 1/2 cup, then follow the same small-bag sealing step.

My mixture didn't turn into ice cream after 10 minutes—what should I check or try next?

Check that the large bag was about half full of ice with about 4 tablespoons of rock salt sprinkled over it, confirm the small bag was tightly sealed with air pressed out, and if needed add more ice and salt and shake vigorously for longer.

How can we adapt this activity for younger kids or older kids?

For younger children have an adult do the measuring and the vigorous 10-minute shaking while they stir, taste, and observe, and for older kids let them measure the 1/2 cup milk and 1/4 teaspoon vanilla, seal the bag themselves, time the shaking, and record temperatures or results.

How can we extend or personalize the ice cream activity after making it?

After scooping the ice cream into bowls you can personalize it by stirring in mix-ins like chocolate chips or fruit, make separate small bags with different flavors to compare outcomes, or turn it into an experiment by timing how different salt amounts affect freezing and sharing the results on DIY.org.