When to display node's teaser or full node... GET Variables?

I'm trying to display a node's teaser via iframe and want to display the full node elsewhere on the site.

In normal php I would do this by using a GET variable in the iframe's URL and process that variable to determine whether to display the full node or teaser. In drupal, I'm not sure how to accomplish this. Seeing as how template files are called indirectly.

How can I tell the node template when to display the full node and when to display the teaser?

I saw the sample code from a node.tpl.php file on http://drupal.org/node/53464:

if ($teaser) {
  // node is being displayed as a teaser
  // Anything here will show up when the teaser of the post is viewed in your taxonomies or front page
} else {
  //all other cases
  //Anything here will show up when viewing your post at any other time, e.g. previews
}

But in this case, how do you send the $teaser variable to the template file?