This has quickly turned into a new favorite activity for my son. He loves hands-on, messy stuff! A friend did this at our toddler art group this summer. Add a bit of food coloring to vinegar (you can dilute it a bit with water if desired), and then give the tots droppers/pipettes to squeeze and add the colored vinegar to the soda. They usually do the dropper method for quite some time before realizing they can scoop the soda directly into the cups of vinegar for more fizz! Then they can pack the soda into the cups like sand at the beach!