I love this one for kids, and adults too: Popsicle Rockets

Here’s what you need to get started:  red juice, blue juice and white juice.
Red is easy, blue can be blue Gatorade or Kool aid, and white juice can be lemonade.

You’ll also need two dozen 3oz paper cups and two dozen Popsicle sticks.

• Begin by having your children mix up the juice if need be.
• Pour two tablespoons of the red juice into the bottom of each cup and place in the freezer for 2-3 hours.
• Remove from freezer add two tablespoons of the white juice and place Popsicle sticks into the center of each cup.  Take care when pushing the sticks into the mostly frozen red juice.  Children can get enthusiastic and push the sticks all the way through the cup. 
• Place back into the freezer for another 2-3 hours.  Pull out and top with white juice. 
• Refreeze until hard.  Peel off paper cups and serve when ready!