The "Shake It Off" singer was a guest star on Fallon's special anniversary Tonight Show on Tuesday, Feb. 17, which marked a year since his debut as the late-night show host.
After talking about Swift's effusive audience participation dance moves (which Williams didn't seem to appreciate during the Feb. 8 Grammy Awards!), Fallon set up the amusing bit by having an emotional moment.
"Every time I see you dancing at events I get a little sad," Fallon told Swift, in his most dramatic acting voice. "For a while there, that was our thing."
"That was a long time ago," Swift replied. "But we really used to go all out. I'm sorry I forgot about that being just our exclusive thing."
"We were famous jumbotron dancers," Fallon continued, before explaining that he'd been in touch with a lot of New York City arenas to collect dance cam footage. And guess what? Fallon and Swift star pretty heavily!
Amid footage of real sports fans going all out in the audience, Swift and Fallon don Knicks, Nets, Islanders, and Rangers gear while showing off their coolest dance moves (and chucking a lot of popcorn about!)
Watch the pair's entire dance-athon in the video above!