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Did Bill Nye (aka The Science Guy) send out a tweet with the text "Are you made of copper, nitrogen, terbium and silver? Because you're a CuNTbAG"? No, the screenshot of that tweet seems to have been manipulated, it is a fake image.

We saw the page "Memes" on Facebook publish an image claimin just that on April 17, 2018 (archived here):

While the chemical symbols for copper (Cu), nitrogen (N), terbium (Tb) and silver (Ag) do indeed spell out "CuNTbAg" if you put them together we found no evidence of Bill Nye having made such a tweet. A search for the relevant part of the tweet combined with Bill Nye's verified Twitter account name returns no results.

It is immediately suspicious that the screenshot of the tweet lacks a date/time or the counter for retweets/likes/comments. This leads us to strongly suspect the image was manipulated.

We found one Twitter account that sent out the quote a few hours ago:

Are you made of copper, nitrogen, terbium and silver? Because you're a CuNTbAG -- Nick fletcher (@nickfletcherrr) April 16, 2018

We also found a reference to a (now deleted, non-cached) 2015 tweet from Bill Nye parody twitter account @bill_nye_tho.

The joke itself has been going around for years as confirmed by a quick Google search.