This activity from Kathy Barbro at artprojectsforkids.org is similar to an old post I have here on abstract drawing. In my post I talked about using all kinds of hard edged, often free from shapes to create abstract drawings and then having the children color them in with good, strong permanent markers.
In Kathy’s post she has the children using geometric shaped cardboard pieces for the children to create similar pieces of work. However, to color them in she has them using non permanent water based markers and then has them drip water to make the colors bleed. She actually wants to colors to run into each other which turns the geometric shapes into organic shapes.
When the paint is dried she then gives them black sharpie markers to have them draw around the organic shapes the running markers created.