Water molecules like to stick together. On the surface where the water meets the air, water molecules cling even more tightly to each other. This is called surface tension. To lower the surface tension of water, you can heat it up or add soap or detergent. These two things cause the attraction between the water molecules to lessen.
Join me over at 123Homeschool4Me.com for a simple surface tension experiment that will help you learn about this concept first hand.