Have the kids help find 3 different sets of 2 similar toys. We had 2 dinosaurs, 2 frogs, and 2 fish. They should be able to fit in your palm. Next, pour a bunch of baking soda into a bowl. I probably used about...1 cup? Slowly stir in some water making sure you don't add too much otherwise it won't form together. Form balls around each individual toy. Place each ball into a small container.
To add some variety, I added cocoa into half of the baking soda mixture. If I had thought about this before I started, I probably would have make them all brown. It smelled good and it was more rock like!
Let it all dry! They should harden up in an hour or two.
Here comes the matching part. I had each of the boys pick two containers that they thought might have the same set of toys inside. Placing both containers inside a deep 9x9 pan, pour vinegar over to slowly dissolve away the baking soda to reveal the toy inside. This is the fun part because it overflows and fizzles! Make sure you have enough vinegar on hand because the more you add, the more it overflows!
If they had a match...they won! If they didn't, have them pick one of the leftover cups. My kiddos didn't get a match at first so they each grabbed a cup, dissolved it away, and then both quickly found their match.
This was a fun activity that both the 6 and the 4 year old liked. You could definitely make it easier or more difficult depending on your ages.
Hope you have fun with it!