Building a large Drupal site may be a complicated task, but building small parts of it as prototypes helped us to build Go Ask Alice!, a flagship site of the Columbia University in the City of New York. If you are about to build such a large site, then continue reading why, what and how you should build using prototypes. (This is a transcript of my part from our presentation at Drupalaton 2015 with Kristof Van Tomme.)