Just hit the green flag and watch. For a group of 4th grade girls who kept asking if we could do a science experiment. (We will do the actual experiment with actual vinegar and baking soda, too, but the class was nominally about coding.) My dream is to have them think about what is going on inside the bottle in terms of electrons, protons, carbon dioxide, etc.
The cork, the picture of the baking soda, and the bottle came from the Internet and should be credited or replaced by original artwork -- I was throwing this together in 20 minutes. The other artwork is built-in Scratch artwork. I also read about 6 articles about how to get sufficient "time release" for the Baking Soda to enable placing the cork before it fizzles over. I used to do this stuff as a kid (rockets, mainly) but it had been a while! The coding is sloppy -- maybe I'll fix it someday.