On the very first level, after you have grown up, go to where Rodney's dad is located. Once you are there in the diner room where he is found, he asks you to hold L1 and select the Scrap Launcher. Select it, but do not shoot targets yet. Instead, shoot everywhere else, but not at the targets. Use up all your scrap, and Rodney's dad will give you 10 extra scrap.