The one we did was about surface tension (though we didn't discuss that at all). In a bowl we put water and sprinkled pepper in it, then dipped a toothpick in dish soap and put the tip in the pepper water. The pepper all moves to the edge of the bowl. Then we did the same thing with food coloring in milk, and created some cool swirl effects (not any great pics of that). Taipan and Simi thought it was pretty cool, and each repeated them 3 or 4 times.