Sky Has No Limits...! So Do I....!

We all aim to reach "the sky" in every field of our interests. Also I have heard in many public speeches, somebody exclaim "Sky is the limit...!".

But what is that "sky...?" "Is there really an object called Sky?".. "Does it have dimensions...?"
By looking at the relative positions of Sun and the Sky we infer that the Sky is much farther than the Sun..!
When we rocket up from the surface of Earth, we cross many layers of atmosphere, and reach the International Space Station or The Moon. But where does the Sky start?
When can we literally reach the Sky? Scientists have disclosed the distance between The Sun & Our Planet Earth. But none have revealed the distance between The Sky & Our Planet Earth.

The fact is that there is nothing called as Sky. Above the surface of the Earth, we only have air through certain distance, then comes the clouds and after that literally "nothing". The air above the surface of Earth is held towards the Earth by Gravity, as air also has certain mass.
When we fly by an air-liner, we climb through various layers and reach about 30,000 feet above sea level, where there are only traces of air. The air layer lies mainly below the air-liner so that the pressure of air is less & for this reason the cabins of air-liner are pressurized. When we climb at a distance more than 30,000 feet, there is still less air in the atmosphere.

A comparison can be made by comparing ourselves as Fishes which live at the bottom of the ocean of air. But the ocean ends up at the surface clearly seen on the beaches, unlike air in the atmosphere which gradually decreases. Above this there is nothing except the very distant Sun, other Stars & other planets.
So one might wonder "what is the Sky then?"
What ever we see as blue color is due to the refraction of sunlight. The sun light has various spectrum of colors out of which mostly Earth falls under visible blue spectrum. On a dark night, with a clear night atmosphere, one can see nothing except the twinkling stars...!