To make the goo, I started with cornstarch in a container and slowly add water until all of the cornstarch has mixed with water. It will still be a liquid but all of the cornstarch should be mixed with water.
Next I poured the goo into small cups and mixed in food coloring. Some we did 1 color, others we added multiple colors. It is a little difficult to mix the food coloring into the goo, which is good for adding many colors since they won't really mix together.
Then we put the cups in the freezer. We left them over night, so I'm not sure exactly how long it took them to freeze. When we were ready to play we removed the frozen goo and put it in containers to play with (I definitely recommend putting it in a container because it gets messy as it starts to melt).
This is what it looks like as it starts to melt.
Everyone (including me) had so much fun playing with the goo. It was fun to watch it melt, and to move the melted goo around. Plus once it as melted you can still play with it (and probably would be able to refreeze it but we didn't). Once ours had melted the colors really started to mix together, which was great to use to review our colors and what colors we can use to make new colors.
I'm linking this science activity up with Monday Made It at 4th Grade Frolics!