Both cups started out purple, and changed colors as citric acid and baking soda were added. Then the purple cup on top was both red and blue combined.
Carson got a science experiment kit from his Aunt Andrea for Christmas, so we pulled it out today and did an activity with acids and bases. Tucker looks like he's totally into it in these pictures, but they were all taken after the experiment was over. He wanted to touch everything, of course, and got mad when I told him 'no'. But Carson and I had a good time watching the bubbles and colors.