Facebook spies on us all the time, and, by extension, so do the companies it owns like Instagram and WhatsApp. Now, one app is turning the tables by letting you use all that data they’re quietly collecting to spy on your own WhatsApp friends and contacts.

Chatwatch takes advantage of WhatsApp’s public online/offline status feature, which lets your friends know when you’re readily available to talk. Using this data, the app can tell you how often your friends check WhatsApp. It can even estimate what time they go to sleep and wake up each day.

Even more impressive (and invasive), Chatwatch can combine WhatsApp data from two of your contacts to make an educated guess if they’ve been talking to each other. Basically, if two of your friends were both online for the same consecutive five minutes in the wee hours of the morning, there’s a decent chance they were messaging each other.

It’s a creepy new trick that the app’s developers hope will bring more attention to how Facebook handles our data, along with how other companies access and analyze it. It’s also pretty likely that WhatsApp will find a way to block Chatwatch soon. So if you want to spy on your friends expose Facebook’s privacy issues, you should try it soon.

One small catch: Chatwatch isn’t free—it’s actually kind of expensive. For $1.99, you get the ability to spy on two of your contacts for a week, so choose carefully. The developers say this money will go towards maintaining the app and funding an anti-Trump political non-profit.

Chatwatch initially launched on iOS but it was later pulled from the App Store by Apple for unspecified reasons. It’s currently available for Android, and the developers are currently working on a web-based version as well.

Updated March 29, 3:41 pm ET: Updated with info on availability after Chatwatch was removed from the App Store.