Digital signage can interact with mobile phones. Using SMS messaging and Bluetooth, some networks are increasing the interactivity of the audience. SMS messages can be used to post messages on the displays, while Bluetooth allows users to interact directly with what they see on screen. In addition to mobile interactivity, networks are also using technology that integrates social and location-based media interactivity. This technology enables end users to send Twitter and Flickr messages as well as text messages to the displays. Some signs use 3D displays that operate using a technology called autostereoscopy, which allow the viewer to see a 3D image without using special glasses.