Following a tweet by Trump which told the late night show host to "be a man," Fallon responded to the furore in the opening monologue of his show on Monday night.
"When I saw that Trump had insulted me on Twitter, I was going to tweet back immediately, but I had more important things to do," Fallon joked. "Then I thought, wait, shouldn't he have more important things to do? What are you doing? You're the president. Why are you tweeting at me?"