The question was about age.

It came at a January event in Dubuque, Iowa, when a woman asked Senator Bernie Sanders how she should respond if someone said he was too old to be in office.

“Is experience important in a crazy world like today? Yeah, I think it’s of some importance,” he said flatly. He waited a beat for the crowd to get it. “That’s a joke!” he exclaimed, when it was apparent few did. “I think it’s very important!”

This happens a lot.

Mr. Sanders has learned a few things on the campaign trail. He has learned how to attack President Trump by rattling off a series of unflattering descriptions. He has learned how to get people to talk about themselves by asking them to share their struggles with health care. He has also learned how to deliver a punchline.

And as he has campaigned in the 2020 race, briefly as a front-runner, then as the only thing standing in the way of former Vice President Joseph R. Biden Jr.’s primary coronation, he has emerged as something very different from the publicly gruff, politically unchanging character he has always played: He has actually become a bit playful.