FaceApp is provoking a combination of delight and repulsion among the huge number of people using it to turn themselves and others older.
The app uses artificial intelligence to change your photos – changing it so that users look young or older, or swapping their genders, for instance.
But the terms of the Russian-owned app have led to a number of warnings about what it could actually be doing with your photos.
When users submit a photo to the app to change how it looks, it makes its way onto FaceApp's servers. And it is not entirely clear what is happening when it does.
FaceApp has to select and upload the users' photo to ensure that it can be altered. The changes use FaceApp's artificial intelligence tools, which run on its servers, and so the photo must make be given over to the app.
But you might be giving over more than you realise. Numerous people have pointed to the fact that those photos can be used by FaceApp – and not just to make you look different.