Certain videos on YouTube, that contain either nudity, explicit language or violence, are put under the 18+ category and you cannot watch these videos* unless you sign-in to your YouTube /Google account and click Yes to the following warning:

This video or group may contain content that is inappropriate for some users, as flagged by YouTube’s user community or as determined by the video uploader.

To view this YouTube video, verify you are 18 or older by signing in or signing up (link - http://www.youtube.com/verify_age?next_url=)

PS

If you don’t have a Google account or if don’t want to log in to YouTube just for watching a video clip that has been flagged as potentially NSFW by other viewers on YouTube, here a trick - switch to the full screen mode.

All you YouTube video URL look something like this:

https://www.youtube.com/watch?v=Iq3zo432sAU

If you replace “watch” in the YouTube URL with “watch_popup,” the adult only video will immediately play inside your browser screen without any restrictions.

The modified YouTube URL, that will play without the adult warning, will therefore be:

https://www.youtube.com/watch_popup?v=Iq3zo432sAU

Related: YouTube Video not available in my country

There’s another trick as well. Instead of adding watch_popup, take a ID of the YouTube video and replace it in the following URL

https://www.youtube.com/embed/VIDEO_ID_HERE

The last option will however not work for videos that have the embed option as disabled.