What is Windows?

[#1]

My 9 year old Grandson has a laptop running Windows 8.1 which he is happy with.
The other day he spotted the Windows 10 upgrade icon and asked what it was.
I made the mistake of telling him it was a free upgrade to Windows 10 to which he replied with the question "what is Windows 10".
This led to "what is Windows" and "what is an operating system"
My every attempt at explaining led into technicalities and we both quickly gave up.
How do you explain to an intelligent 9 year old what an OS is....and keep it engaging

[#2]

Examples of popular modern operating systems include Android, BlackBerry 10, BSD, Chrome OS, iOS, Linux, OS X, QNX, Microsoft Windows,[3] Windows Phone, and z/OS. The first eight of these examples share roots in UNIX.

[#3]

I don't think he's quite ready for that Foxy

[#4]

Some people would answer the question of "What is Windows?" with this:
"Microsoft Windows: A collection of 32bit extensions and a graphical shell for a 16bit patch to an 8bit O.S. originally coded for a 4bit microprocessor written by a 2bit company who cant stand 1 bit of competition."
I'd just say, look here:
https://en.wikipedia.org/wiki/Microsoft_Windows

[#5]

Unless its X-Windows, common on UNIX-like computer operating systems. 

[#6]

It's like the engine that runs your car. Without an engine, you aren't going anywhere 

[#7]

An operating system is a set of instructions that tells the computer/engine how to work.  Once you have the basics of an operating system then you can write computer programs that tell the computer how to perform specific tasks.  Windows is the operating system made by a company called Microsoft (and it actually comes with a large number of programs thrown-in).  You could, if you wanted, take the same hardware device and give it a different operating system (Linux, for example) in which case you would need a completely different set of computer programs to run on it.