Create Alert When iFrame is clicked

I am trying to get this so when I click inside an iframe whose source is from "site1.com" it shows an Alert. I did place the code after the iFrame and it does nothing.
Here's my code:
Code:

<script type="text/javascript">  
function showAlert(){
	alert("iframe clicked");
}
var elements = document.getElementsByTagName('iframe'); 
{ 
if(elements.src.indexOf('site1.com')
{ 
elements.onfocus = showAlert();  
} 
} 
</script>

Code (markup):

Thank you

 

not possible unless you can change what's inside the iframe. js doesn't permit it crossdomain for obvious security reasons