jQuery's `this` and `this` in the function

Question

I've code like the following:

function tell(){
  console.log(this==window);//true
  console.log($(el).html());
}

$('.elem').click(function(){
  el=this;
  console.log($(this).html());
  tell();
});

I want to know why tell()s this is not this from the click event handler anonymous function?