In the company's last earnings call, Apple Chief Executive Tim Cook said the company's Apple Watch and AirPods headphones generated $10 billion in revenue across the past four quarters….The key is the portion of the company's revenue that comes from services—a piece of the pie that's been growing even faster than wearables sales. In 2017, "services" was already a $30 billion business at Apple, and in the company's most recent quarter, it accounted for nearly $10 billion…. All of Apple's wearables are not only compatible with Apple's services, but also seem designed to enhance those revenue streams. Consider the fact that a cellular-connected Apple Watch Series 3 can stream only Apple Music, or that Siri is the only smart assistant you can summon directly from Apple's AirPods.