Former Vice President Joe BidenJoseph (Joe) Robinette BidenTrump stokes new unlikely feud Sanders on 2020 White House bid: 'We're looking at it' Dems wonder if Sherrod Brown could be their magic man MORE said Thursday at the University of Pennsylvania that President Trump needs to "grow up" and "stop tweeting."

When asked by a student at the launch of the new Penn Biden Center for Diplomacy and Global Engagement, "If you could give president Trump one piece of advice what would it be?" Biden promptly replied, "Grow up." 

Biden continued that were he the president, "I would stop tweeting and start focusing."

"The words of a president matter. They have enormous, enormous, enormous, reverberating sounds around the world," Biden said. "Every time a U.S. president speaks and says something, leaders and people around the world try to dissect what he means because it matters so much to their security, threats they face, whatever it is. So, you can't loosely make assertions without limiting your ability to lead."

Biden also said he doesn't think Republicans will be successful in pushing tax reform, which GOP lawmakers have indicated is now their top priority. 

"I mean that in substantial part because, in fairness to Trump, I don't believe Trump ever thought he would be President Trump. I don't think he thought he was going to be president," Biden said, stressing that in the past Trump even supported Biden when he ran as a Democrat for the Senate. 

"I think that by the time it got around to realizing he just might win, he had not taken the time ... to put together a transition team. ... And so he was at an enormous disadvantage when he won," he added. 

"One of the reasons I predict there will be no tax bill is there's not enough people in the Treasury to handle the calculations and strategy," Biden said, echoing reports that Trump has left a large number of government positions unfilled.

"I was told at least three weeks ago you could walk down the halls of the State Department and holler and no one would hear you," he joked, "because no one was there."

Updated: 7:42 p.m.