Detecting mouse over event

I was trying to register an event handler with a div's onmouseover event, there's something interesting that I found.

div.onmouseover = alert('yatta!');
- this event will be triggered every time the div is appended to another element (weird?), because u cant trigger a function with params with this statement.

div.setAttribute("onmouseover", "alert('yatta!');");
- this event will be triggered only when the mouse is over the div