Crosby, Stills and Nash with Jimmy Fallon

Stephen Stills, Graham Nash and David Crosby perform with host Jimmy Fallon as Neil Young on the Tonight Show Starring Jimmy Fallon on July 7, 2014.

Douglas Gorenstein/NBC

Charli XCX may have appeared on "Late Night WIth Seth Meyers" last night, but the No. 1 song she guests on was given a ridiculous cover treatment by none other than Jimmy Fallon.

If you just stumbled upon last night's "Tonight Show" while channel surfing, you might have thought it really was Neil Young (a 40-year-old Neil Young, that is) folking out the opening verse of Iggy Azalea and Charli XCX's "Fancy" on acoustic guitar and harmonica. But it was Fallon, immersed in his chillingly accurate '70s Neil Young act.

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Following a little harmonica solo, Fallon is joined onstage by the real life David Crosby, Stephen Stills and Graham Nash, giving birth to a new supergroup one might call CSNF. The quartet harmonizes and strums through another bit of the current top Hot 100 track, before ending with a little CSNY chant.

Fallon has been open about his Neil Young fandom for a while. In fact, Young himself joined him two months back on the "Tonight Show" to press an instant vinyl of a live performance with Jack White.

Iggy Azalea’s song “Fancy” was spotted charting on the Twitter Trending 140 chart after Fallon’s performance on July 8 at 12:35 am on NBC. The song stayed on the chart, fluctuating between No. 3 and No.15 as the most talked about song on Twitter. Around 11 am this morning, the song charted at No. 1 on the Trending 140.

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