Before polls closed in Michigan’s Democratic primary, former Vice President Joe Biden told a factory worker in Detroit he was “full of shit” during a brief but heated exchange over guns.

The verbal altercation occurred while Biden, the Democratic frontrunner, was touring an auto plant during a scheduled campaign trip. At one point, the worker contends that the former vice president is “actively trying to end our Second Amendment right.”

Without wasting a moment, Biden responded: “You’re full of shit.”

“I support the Second Amendment,” he added quickly. But “the Second Amendment–just like right now, if you yell ‘fire,’ that’s not free speech,” he went on, before adding additional context. “I have a shotgun, I have a 12-gauge," he said.

“You’re not allowed to own [just] any weapon. I’m not taking your gun away at all. You need 100 rounds?”

This was hardly the first tense moment Biden has caused on the trail, with several occurring before voting stated and well before his frontrunner status was cemented.

Back in early December, for example, it was day five of Joe Biden’s bus tour through Iowa when the malarkey hit the fan.

A tense exchange with a voter led the former vice president to call him a “damn liar,” question the voter’s IQ, insult his appearance, and challenge him to a feat of strength.

It will come as no surprise to even casual Biden-watchers that this is not the first (nor will it be the last) time he’s found himself in this situation—particularly when it comes to questioning the intelligence of the target of his ire.

Biden’s preoccupation with IQ—or intelligence quotient—goes back to his childhood when his mother would reassure him that just because he stuttered didn’t mean he wasn’t smart.

“Joey, you’re so handsome. Joey, you’re such a good athlete. Joey, you’ve got such a high IQ. You’ve got so much to say, honey, that your brain gets ahead of you,” he recalled his mother telling him as a young boy in his memoir, Promises to Keep.

But what began as a mother’s comfort evolved on the campaign trail as a cudgel—one of many Biden has reused over the years when he’s triggered by a feisty voter or a circumstance he found unacceptable.

So what situations cause Uncle Joe to go off the rails? The Daily Beast has put together a list of possible scenarios that trigger Biden to request an intelligence quotient test and/or push-up contest.

Question his academic integrity

Longtime observers of New Hampshire primary politics may recall a fiery exchange over 30 years ago, when then-Senator Biden insisted he was much smarter than a Granite State voter, who asked a question at a campaign event about parts of his academic record, which was at the time the talk of part of his first presidential campaign.

“I probably have a much higher I.Q. than you do, I suspect,” Biden said in February 1987 at a campaign stop in Claremont, a rural town in New Hampshire, according to an account in The New York Times.

At the crux of the flash dispute was the idea that Biden may have stretched the truth about part of his higher education background, prompting a voter to ask what law school he attended and where he placed in the class. Answering the question, Biden detailed a lengthy personal account of his academic résumé, ultimately saying he’d be “delighted” to compare IQs with the voter.

“I went to law school on a full academic scholarship, the only one in my—in my class that have a full academic scholarship,” he started. “In the first year in law school I decided I didn’t want to be in law school and ended up in the bottom two-thirds of my class, and then decided I wanted to stay, went back to law school, and in fact ended up in the top half of my class. I won the international moot-court competition. I was the outstanding student in the political science department at the end of my year. I graduated with three degrees from undergraduate school and 165 credits—I only needed 123 credits,” he said.

He concluded by saying: “And I'd be delighted to sit down and compare my IQ to yours if you'd like Frank.”

He later confessed in Promises to Keep that he “lost his temper” that day in Claremont.

“ What I’d said about my academic achievements was just faulty memory or lack of knowledge. I hadn’t remembered where I finished in my law school class,” he wrote. “I hadn’t cared. But to say ‘Wanna compare IQs?’ was so stupid. All of it was my fault, and I didn’t want to compound the mistakes.”

Say something negative about his family

On Thursday, a self-identified Independent farmer from Iowa unintentionally test-ran what could possibly be the most explosive line of questioning yet: asking Biden about his family.

The man, who Biden generally calls “Jack,” with some exceptions, is a “damn liar” and “too old” to caucus for him in less than nine weeks time, according to the former vice president.

“Get your words straight, Jack!” Biden yelled in response to Jack’s assertion that his son was engaged in nefarious activity in Ukraine, a claim that’s been debunked by multiple credible news outlets, but that the Trump campaign nonetheless views as a top vulnerability of his campaign.

“We all know Trump has been messing around in Ukraine,” the man said, moments before moving to Biden. “You on the other hand sent your son over there to get a job and work for a gas company and [who] had no experience in natural gas,” he said. “You’re selling access to the president just like he is.”

At that point, Biden jumped in with vigor: “You’re a damn liar, man. That’s not true. And no one has ever said that.” He also proceeded to (unintentionally, the campaign contends) call Jack “fat” instead of the word “fact.”

As the exchange escalated, Biden momentarily pondered dropping the subject, thinking aloud: “Let him go. Let him go,” he said. Then came the moment that had been building during the entire exchange, a chance to revert to one of his greatest hits.

“Let’s do push-ups together here, man. Let’s run. Let’s do whatever you want to do. Let’s take an IQ test.”

And it’s not just blood relations that unleash such a reaction from Biden.

A 2014 New Yorker profile describes an altercation Biden had with then-Rep. Anthony Weiner (D-NY) during a Democratic Caucus meeting after they lost the House in 2010.

Following Weiner’s criticism of President Obama’s decision to cut a deal with the GOP on tax cuts, Biden erupted.

“There’s no goddam way I’m going to stand here and talk about the President like that,” Biden said, according to the report.

Trumpian Behavior

This one is not an exact science. So far, there’s little evidence to suggest that merely adopting similar Trumpian tactics would provoke Biden enough to reflexively go for the jugular. But it’s worth examining a basic blueprint.

In a behind-the-back exchange with Trump — the general election opponent he has styled nearly every aspect of his primary campaign around defeating — Biden suggested he would “beat the hell out of” the president if they were in high school.

Biden was referencing Trump’s comments that surfaced during the now infamous “Access Hollywood” tape during the 2016 election, in which the president bragged about grabbing women’s bodies without their consent.

“They asked me would I like to debate this gentleman, and I said no. I said, ‘If we were in high school, I’d take him behind the gym and beat the hell out of him,” Biden said in a speech in Miami in March 2018.

It didn’t take Trump long to fire back.

“Crazy Joe Biden is trying to act like a tough guy. Actually, he is weak, both mentally and physically, and yet he threatens me, for the second time, with physical assault,” Trump tweeted. “He doesn't know me, but he would go down fast and hard, crying all the way. Don’t threaten people Joe!”

It’s unclear exactly which Trump-style tactics would be irritating enough to provoke a similar response from Biden. In fact, there’s one indication that it might not work at all. He later walked back the tough-guy talk, telling the hosts of “Pod Save America” that “I shouldn’t have said what I said.”

“I don’t want to get down in the mosh pit with this guy,” he said, months before using the unofficial campaign slogan “beat him like a drum” on the trail. In solidarity, some members of his 2020 team proudly feature tiny emojis of the percussion instrument alongside their Twitter handles.