If he was a little bit older or even on a better day when he was more interested he would have been great. But...
Here's how to do it:
Shallow pan of baking soda.
bowls of food colored vinegar.
dropper or maybe 1/4-1/2 teaspoons.
Drop the colors and watch them fizz.
The set up, mom taking pictures of the cute faces while I prep.
The first few drops.
You can see his intensity and focus. It really would have been great had he been able to figure out the dropper. It wasn't as fun of a project if mom has to do the whole thing for you.
As you can see my colors weren't as vibrant as the original pinner's were but he had fun with the fizziness of it anyway.
The boredom with the dropper was quick so this was the point where he just dumped 'em all in and they fizzed up quite nicely. I promptly sent the whole mess down the drain!
The original pin with this idea is just a picture but you get the idea. I found it here: color fizz