onClick Function "this" Returns Window Object

I've come across a head scratching issue with my JavaScript application. If I write an element like this: <li onClick="alert(this.tagName)"></li> I get "LI." However if I do this: <li onClick="foo()"></li> Where "foo()" is: function foo(){ alert(this.tagName); } I get "undefined." I am away how "this" is supposed to work in regards to attached functions. But, I am baffled because "this" is not pic