Amazon has started selling a wrist band that tracks workouts, sleep and body fat. To measure body fat, Amazon will ask users to scan their body with their smartphone cameras while wearing boxers or a sports bra with bike shorts.

The fitness tracker will give the company a wealth of new information about its customers. The microphones in the tracker will help it listen to how people talk, which could make its voice assistant Alexa smarter by detecting moods and providing better responses.

Data has been a concern with other fitness trackers. Amazon said health data is encrypted so others can’t see it, and users will have the ability to turn off the microphone.

The fitness tracker is called the Amazon Halo and costs $100. There’s also a monthly $4 charge to use all its features. Those who don’t pay up can still track steps, sleep times and heart rates.