Avoid shared variables among notebooks because "globals"?

Questions
1

Hi there,
I’m pretty new to Mathematica. Usually when I use Clear[X], variable X will lose its value and turn blue. But sometimes Clear[X] makes X lose its value but not turning blue and staying black. This is confusing, what is going on?
And what is Kernel? I think things are related some way.

Edit: I was using K as variable and I discovered it’s a special one. I also discovered that variables are shared among notebooks because “globals”. How can I make local variables or how can I delete global ones?

Thanks.

2

The uppercase letters C,D,E,I,K,N,O are used by the system, and will always be black. You had better avoid them as your own variable names. In general, it is advisable to start your variable names with a lowercase letter, which is guaranteed to avoid any conflict.

3

Thanks. Do you know if there’s the possibility to use “local” variables? I’d like to have variables seen only by the notebook they are defined in.

4

I’d like to have variables seen only by the notebook they are defined
in.

is-there-a-way-to-separate-variables-between-multiple-notebooks

5

Thank you.