What is the best way to answer a child’s “why is the sky blue/grass green?” question?

I'll leave the grass question to another Hubber but here's one way to handle the sky question.

It's something scientists call "Rayleigh Scattering." You can read more about that at How Stuff Works.

To explain it to a chlld, you have to flirt with the concepts of molecules and light. But to make it understandable, it's best to say that everything in the world is made up of billions and billions of molecules, which are tiny little building blocks - different chemicals stuck together by electricity. They are so small that you can't see them, even with the best microscope. But a lot of smart people figured out that they exist - and we know that just about everything around us is made of molecules. Water, soap, sugar, bread, the dog, even us!

Next, have you ever seen a rainbow? All those colors are in sunlight, all the time. Red, orange, yellow, green, blue, indigo (which is dark blue) and violet or purple. But we only see the yellow/white light of the sun - until it passes through something that changes it and breaks it into those hidden colors. A rainbow is made when sunlight passes through a whole lot of raindrops - or sometimes a piece of glass, like if you've ever seen the dangly bits of a glass chandelier make rainbows on the wall.

Air - like almost everything else around us - is made up of billions of air molecules. When light passes through them, it scatters - what scientists call "refraction." the way raindrops splits light into all its colors and makes a rainbow, air does a similar thing. But the air molecules are just the right size and shape to makes the light look blue. Air does not do such a good job at uncovering the other rainbow colors, so blue is what we see when we look at air lit by sunlight (the sky.)

You can demonstrate this. Wait for nightfall because you have to do this in the dark. You'll need:

• A clear glass with no patterns or markings on it.
• Water
• Milk
• A flashlight

Fill the glass about 2/3 with water. Now add a teaspoon of milk and mix it up. The milk plays the part of the air molecules, floating around the water glass, just like air molecules float around the atmosphere.

Now turn down the lights and shine the flashlight into the glass. Move the flashlight around, shining it from different angles and directions and eventually you'll see the liquid take on a faintly blue hue - even though the water is clear and the milk is white. The milk molecules scatter the light and change its color, just like air molecules do in the sky. You might even make it red - which is a representation of how dust in the air makes the sunset and sunrise red.

If it does not work at first, you can try adding more milk, a half-teaspoon at a time. You can get many more details on this experiment, blue skies and the refraction of light here.