Follow these directions from homeschool journal:
Tear a paper towel into a square that measures about 5 inches by 5 inches. Put 1 1/2 tablespoons of baking soda in the center of the square, then fold the sides of the square in toward the middle, and then the opposite sides in toward the middle, with the baking soda inside. This is your "time-release packet." Pour into your plastic bag, 1/2 cup of vinegar and 1/4 cup of warm water.
Now here's the tricky part. You need to drop the time-release packet into the vinegar and zip the bag closed before the fizzing gets out of control. You can zip the bag halfway closed, then stuff the packet in and zip the bag closed the rest of the way in a hurry, or you can put the time-release packet into the mouth of the bag and hold it up out of the vinegar by pinching the sides of the bag. Zip the bag closed and then let the packet drop into the vinegar. One way or another, get the packet in the vinegar and zip the bag closed.
ADD THE VINEGAR OUTSIDE!! MY HUBBY DID IT IN THE KITCHEN THINKING HE HAD A FEW SECONDS TO GET OUTSIDE AND HE WAS ABOUT 4 STEPS FROM THE FRONT DOOR. THE BOYS GOT A KICK OUT OF THAT!!!!:))
They had a lot of fun with this! Nice and noisy and if you add food coloring to the vinegar first it gets some great color foam! My husband is like a big kid when it comes to this stuff so after they tried this they tried to explode a water bottle with it, and medicine bottle. Neither worked. The water bottle did get a ton of pressure but it had enough air to leak some out and not explode it. They had fun trying though! Tomorrow we will get the volcano action going!