Passing a local variable from one function to another

I am a beginner in JavaScript and wanted to ask the following: I have two simple functions and was wondering if there is any way to pass a variable value from one function to another. I know I can just move it outside the function to be used in other functions as well but just need to know the way how I can have one local variable and manipulate with it in my second function. Is this possible and how?

Here is some code:

window.onload = function show(){
    var x = 3;
}

function trig(){
    alert(x);
}
trig();

The question is: how do I access variable x (declared in the function show) from my second function trig?