Duo, a video chat application developed by Google, has a "knock knock" feature that allows users to choose whether or not to start a video call.

Starting a Duo video chat is incredibly easy and fast. Launch the Duo app and the screen will display a preview of the user's own video, as well as a big red button to tap to start a video chat. Duo is compatible with iOS and Android devices.

Simple interface - Call any contact with just one tap. The system will present you with a simple video interface to chat with each other.

Cross-platform service - Initiate video calls to all your friends from your smartphone with just one app.

When it comes to video chat apps, Google Duo is one of the best. It's simple and free to use, no clunky downloads necessary! You just need to sign into your Gmail account and you can video chat with anyone else who has this app installed. Once you're connected, the audio and video quality is pretty darn good — better than other apps we've tested at any rate. The app will switch from landscape mode (horizontal) to portrait mode (vertical) automatically depending on which way you hold your phone. To switch your call to another device, simply "OK Google" and your call will automatically move to the other device. This is a great feature and lessens the need for a dedicated video device like a computer or tablet. One thing we do wish Google Duo had was the ability to cross-platform. Truth be told, our team is pretty divided on this with some people preferring iPhone users be stuck with Apple's own FaceTime app, with others wishing it could work on most any mobile platform. With that said, if you are reasonably confident you don't have compatibility issues then you might want to give Duo a shot. If you can't get your hands on a Google Duo invite, there's a bit of a workaround. Head over to a friend's phone and install the app from them, then sign into your account from there. Your friend can cancel their Google account after you have set up the app on your phone. So, have some friends over for some old-fashioned video chat! If you use Google Duo more than once, consider leaving a review here !