Those sensors are just sitting there doing nothing, or performing mundane tasks like giving you turn-by-turn directions or turning off your screen when you're yakking on the phone.

But what if you could use phone sensors to their full potential?

Tiny instruments inside your phone can tell where you are on the surface of the planet. Sensors can also detect and identify nearby Wi-Fi routers and Bluetooth devices. They can measure light and sound, as well as motion, direction and "attitude" (the position of the phone -- which way it's facing). They can tell if something is near the phone or not. Some phones can detect ambient temperature and other environmental facts.

Smart software with access to all the data gathered by these sensors, combined with an Internet full of information, could figure out all kinds of things about you.

Everybody knows that big companies like Google, Apple and Facebook want to harvest cellphone-generated data and use it to serve up virtual personal assistants with a side order of contextual advertising.