Water Has a Skin

• Before beginning the science experiments, explain to your children or students that water has a skin. Point out that water's skin is very weak, which is why water tends to flow all over when you spill it. Talk about how a person's skin is strong in comparison, which means that people don't flow like a liquid.
  
  Tell the children that the skin on water is called "surface tension" and that it's possible to see it in operation if you do a few experiments. This introduction should get the children curious and eager to learn.
  
  

Balls of Water

• Lay some wax paper down on a table and get out several bowls of water along with bottles of liquid food coloring. Allow the children to have some fun tinting the water using any colors they please. Tell them that the water isn't dense enough to keep the food coloring on the surface of the water. Then use an eyedropper to gently transfer a drop of colored water onto the wax paper. It will roll across the paper and form a ball which holds its shape.
  
  Tell the children that surface tension is holding the drop together, keeping the water from flowing. Let them create their own balls of water in various colors. Some children will probably add drops to existing drops until the ball of water breaks and begins to flow. Explain that water's surface tension is so weak that it can't hold when the ball of water becomes too large.
  
  

Another Way to Break Surface Tension

• Use another eyedropper to transfer a drop of liquid dish soap onto one of the balls of colored water. The water will immediately run out into a puddle. Explain that the soap causes the water's skin to break. Allow the children to experiment by dropping soap onto the other balls of water.
  
  Explain that this property of soap is what makes it able to wash clothes and dishes so well. By breaking the surface tension of water, the soap makes the water more able to flow and get things clean.
  
  

Dancing Colors

• Pour milk into a bowl and ask the children to predict what will happen when drops of food coloring are dripped into it. Allow them to transfer drops of food coloring to the milk, and explain that milk, just like water, has surface tension. Show them how the milk is denser that water, so it holds the drops of food coloring in neat circles on the surface.
  
  Ask the children to predict what will happen when a drop of soap is added to the bowl. Let one of them add a single drop, transferring it to a clear area of milk, not onto one of the drops of food coloring. The colors will begin to swirl and mix, seemingly on their own. Explain that the soap has changed the surface tension of the milk and the colors are now able to flow, appearing to stir themselves.