There are questions that are meant to pose deep philosophical thoughts; to put one in a Zen frame of mind.
I don't see it.
What is the sound of one hand clapping?
Open and close your hand quickly, so that your fingers smack into your palm. The Simpsons answered this one years ago. One hand clapping is the same sound as two; just quieter.
If a tree falls in the forest; but nobody's around to hear it; does it make noise?
No. When you hear something; the air carried the vibrations to the bones of your inner ear. The vibrations hit these small bones and are processed by your brain as noise. Different vibrations in the air make different noises. This is why there is no sound in space. If a tree falls in the forest and nothing records the vibrations; or there is nobody around to hear it; there is still that vibration; but with no ears to pick up the vibration and brain to process; there is no sound.
Why is the sky blue?
Earth's atmosphere scatters light from the sun. Depending on the composition of the atmosphere; the length and angle of the light; the atmosphere diffuses it at a certain wavelength; which our eyes interpret as blue. This is why you get different colors at sunrise/sunset.
How long is a piece of string?
Twice the distance from the center to either end. This is true of a string of any length.