NBC NEWS -- Google can see a future where it sends ads just about everywhere, including car dashboards, watches and even refrigerators.
In a letter sent to the Securities and Exchange Commission disclosed on Tuesday, Google painted a picture of a connected world filled with ads:
We expect the definition of "mobile" to continue to evolve as more and more "smart" devices gain traction in the market. For example, a few years from now, we and other companies could be serving ads and other content on refrigerators, car dashboards, thermostats, glasses, and watches, to name just a few possibilities.
Google is just one of many companies diving into the "Internet of Things," the trendy term for connected appliances and other everyday objects.
The company did not immediately respond to a request for comment from NBC News.
"Today, advertising subsidizes our email, search, Google Drive and lots of other 'free' things that we use," Frank Gillett, a principal analyst at Forrester Research, told NBC News. "The question is, 'How far will that subsidy model go?'"
Google could, for example, sell Google Glass at a discount - provided users don't mind an occasional ad popping up in front of their face.
It's possible that the letter, filed last year, could simply be a sign of Google's reluctance to disclose its mobile advertising revenue to the SEC on the grounds that the term "mobile" is complicated, as opposed to a blueprint for the future of its business.
Whether or not Google thinks it's a smart idea to serve up ads on a refrigerator, the company has been swiftly moving towards a future where that is possible.
In 2013, its technology was featured in the T9000. No, that is not a robot from the "Terminator" movies. It's a smart refrigerator from Samsung with touchscreen interface powered by Google's Android operating system.
In January 2014, Google bought Nest for $3.2 billion. The company became a press darling after debuting a smart thermostat with the ability to "learn" from the behavior of its owners and adjust the temperature accordingly.
That same month, Google announced the Open Automotive Alliance with Audi, GM, Honda and other automakers, an industry group committed to developing Android-powered dashboards.
Google has also been pushing to get its software on people's bodies. Google Glass, the company's $1,500 smart glasses, just went on sale to the general public last week.
This summer, Moto and LG are set to release new smartwatches running Android Wear, which will let wearers do things like check their email and get real-time fitness notifications and directions on their wrist.
Google has the ad platform. It has loads of user information gleaned from services like Gmail. And there are not many technological barriers holding back the Internet of Things.
There were 13 billion Internet-connected wireless devices in 2013, according to Cisco, which estimates there will be 50 billion by 2020. Some experts believe "smart homes" could become common in as little as five years).
The problem for Google is that car dashboards are not the same as Web browsers - people are not going to be too happy if an add pops up over their speedometer.
"The tricky part is a human problem, not a technological one," Gillett said. "What is the line between helpful and annoying?"
Instead, he said, he expects Google to move away from the kind of ads that people see in their Gmail, and towards a world where smart devices serve up "useful, personal suggestions" depending on the situation.
For example, an app running on a smartwatch or Google Glass that lets users know when the subway is coming could give users the option to summon a car from Uber - much like the Google Maps app already does - if the train is running late.
"At the heart of Google's business model are relationships," he said, "and you don't want to drive people out of those relationships."