Further to Andrew Jones 2 post
I read the article and watched the linked video and even followed a link in there to someone's blog about this (more of a fan-squee note than anything useful). The article says:
"The Rickmote Python configuration available on Github was prepackaged with Astley's internet-wrecking hit set to loop."
Does this mean that the device contains a stored video (possibly a few tens of Mb) that it sends to the Chromecast as a continuous locally sourced video stream?
Does the device contain the stored Internet address of the YouTube video, with some parameters causing loop-play and force the Chromecast to connect to it via the Chromecast's 'normal' internet connection?
Does the device have its own internet connection and obtain the video feed from this connection to send to the Chromecast?
The exact method is not clearly explained.