Why does 'this' in a function display window object where as 'this' in an argument displays the object from the function was invoked?

Why does 'this' in a function display window object where as 'this' in an argument displays 开发者_高级运维the object from the function was invoked?

function show(x){
 alert(x)//displays [object HTMLDivElement]
 alert(this)//displays [object Window]
}

<div onClick="show(this)">123</div>

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Because the function is defined in the global scope, so "this" will refer to the window object.