Jimmy Fallon/Crosby, Stills & Nash

This is one of those things you don't know you need until you see it: Jimmy Fallon came back last night from his two-week vacation, and he invited Crosby, Stills and Nash to his show to sing Iggy Azalea's "Fancy."  With himself playing Neil Young, of course.

Fallon's Young impersonation is spot-on, and to their credit, the 60s harmonizers keep straight faces all the way through as they turned the bouncy pop song into a kind of mournful cowboy ballad.  (No, CSN, we don't know what the words to "Fancy" mean either.)

During their tonight show appearance, Crosby, Stills and Nash--who are touring the country this summer--also performed their classic song "Teach Your Children."  Check out that performance, along with their bit with Fallon-cum-Young, below.