All was revealed via a Twitter exchange between Musk and the father of a girl named Bria in Michigan, who Tweeted a photo of a letter his daughter had sent to the serial entrepreneur/modern-day Iron Man about an idea she had for a creative Tesla advertising campaign.
In the letter, in which Bria declares her love for Tesla and its dedication to clean energy, she proposes a clever and cost-effective way for the brand to begin advertising (Tesla famously doesn't advertise). She thinks Musk should conduct a competition to find the best homemade Tesla commercial, then air it wide and reward the winner with "a year of free Supercharging or a Model 3 Easter egg or something."
Bria sent the letter -- which was actually part of a school project -- to Tesla in the mail, and her dad (a writer covering the auto industry) tweeted a copy of it to Musk, who replied excitedly to say not only that he thinks it's great idea, but that he's actually going to run with it. He didn't respond to Bria's post-script request for a Tesla t-shirt, but it's safe to assume an endorsement from one of the world's most renowned inventors is a much better gift.