Once ALL the liquid was in the bottom cup, they wanted to do it again. Instead of starting over, I just put blue water in the top cut and left the red in the bottom and put in a fresh paper towel. They could see that the red traveled up the paper towel and blue down, but the red never made it all the way up into the cup- however the blue made it down and mixed to make purple.
It was a pretty cool experiment, surrounded by a lot of excitement. Some of them actually watched it for large chunks of time, just watching it slowly move.