I made a poster with WEIRD SCIENTIST at the top.  Then I used the 5 clear plastic cups with water in them.  One cup had vinegar in it and was the special cup.  I then put food coloring in a spoon and covered it with baking soda. Someone would come up and choose a spoon to put in the cup.  It would either turn the water a different color or if they chose the vinegar it would bubble over.  The kids loved it.  Oh, I also made them put on my white lab coat so they could be the "Weird Scientist".  The chart I had on the board had the colors match a song.   It was so easy and the kids absolutely loved it.  Also, I put everything on a cookie sheet so that it didn't make a mess.   It was a blast.