WHAT’S HAPPENING?
You may have noticed that the colored water stayed inside the lines as it moved up the strip. Different materials have different chemical properties that affect how they behave with water. Hydrophilic materials attract water, making contact and mixing easily. Hydrophobic materials repel water, beading up or separating out. In this case, the ink in the Sharpie marker is hydrophobic and repels water, so the colored water cannot cross the lines you drew. This allows you to control how the water flows and where the colors mix. Also, since paper has a porous structure, the colored water can freely flow through the pores.