Adults are the world’s fastest-growing group of toy buyers. In the US alone, they’re spending over $7 billion a year on toys for themselves, like Lego, plush toys, Hot Wheels, Barbie dolls, and Sonny Angel figurines. The world’s biggest toy producers, like Mattel, are taking notice, and have released entire product lines for adult fans. We went to Mattel’s design center in California to see how it makes prototypes for Barbies and Hot Wheels. We also met adult toy collectors around America, from the Maryland man with the world’s most valuable Hot Wheels collection to a New York entrepreneur who buys and sells Sonny Angel figurines.