Google's video chatting app has arrived. Duo is Google's response to other popular video calling options, including Apple's FaceTime, Skype and Facebook's Messenger app. Duo gives a glimpse at who is making the call, helping the recipient decide whether to answer. Google calls this feature, "Knock, knock."

The new app has been released as a free service for phones running on Google's Android operating system as well as Apple's iPhones.

Like FaceTime for iPhones, Duo only requires a person's phone number to connect. Many other services require both participants to have account logins to use their video calling options.

From there, you can begin a video call with a tap. Google says it has built Duo to be fast and reliable, so that video calls connect quickly and work well even on slower networks. Call quality adjusts to changing network conditions to keep you connected - when bandwidth is limited, Duo will reduce the resolution to keep the call going smoothly. For video calls on the go, Duo will switch between Wi-Fi and cellular data automatically without dropping your call.

And if you care about security, Duo calls are end-to-end encrypted.

Google is also is preparing to unveil a new messaging app called Allo featuring a robotic assistant that will suggest automated responses to texts.