Trump was asked about deportations, but Kasich's answer stole the show.
Donald Trump has repeatedly come out with with fiery proclamations about undocumented immigrants and deportations.
But during Tuesday's Republican presidential primary debate in Milwaukee, Trump's bombastic language fell flat when confronted with a more reasonable alternative to mass deportations.
One of the event's moderators, Fox Business anchor Maria Bartiromo, brought up the subject by asking Trump about a controversial topic among Republicans: President Obama's program to grant deportation relief to an estimated 5 million undocumented immigrants, a move that's currently stalled in legal challenges.
When Trump cheered a for the deportation relief measure, Bartiromo asked him if deporting those people would affect the economy.
"We have no choice if we're going to run our country properly," Trump responded.
That's when Ohio Gov. John Kasich jumped in.
"We need to control our border just like people have to control who goes in and out of their house," he said. “But if people think that we are going to ship 11 million people who are law-abiding, who are in this country … to Mexico, think about the families. Think about the children."
He continued: "It's a silly argument. It is not an adult argument. It makes no sense."
The pair sparred some more. Trump cited President Dwight Eisenhower's intense border-security regime in the mid-1950s (dubbed, horrifically, "Operation Wetback" and used to deport American citizens) as a sign of success.
(Note: Trump has touted the program — which led to Border Patrol apprehensions in 1954 — as an example of "humane" immigration enforcement, but scholars who have studied the policy found it . Some deportees were removed by cargo ships and a congressional investigation compared one such vessel — where a riot took place onboard — to a "18th century slave ship," or a "penal hell ship," according to by Columbia University professor Mae Ngai.)
Trump tried to shut down Kasich, saying, "I don't have to hear from this man," but the crowd wasn't having it.
The audience did something rare. They booed.
Kasich stuck to his point and won by being realistic.
"We can't ship 11 million people out of this country," Kasich reiterated during their exchange. "Children would be terrified and it will not work."
Regardless of how you feel about illegal immigration, mass deportations probably don't seem like a sensible or realistic solution.
People come to the United States for all sorts of reasons, whether they're looking for jobs, fleeing a dangerous place, or reuniting with family. Deporting millions of people would mean tearing families apart and leaving businesses across the country without workers.
Many of the people in question have put down deep roots in the U.S.
A released by the Pew Research Center in September found that 4.5 million children born in the U.S. — that's right, citizens — have parents who are undocumented immigrants. When parents are deported, children can face everything from psychological trauma to material hardship.
If you're not moved by the humanitarian reasons, consider the economic impact.
So what would happen if the country tried to deport 11 million undocumented immigrants?
First of all, it would cost a lot. A by the right-leaning organization American Action Forum found that rounding up and deporting 11 million people would take 20 years and cost roughly $400 billion to $600 billion.
And that looks small compared with the economic losses. The same report found that the U.S. gross domestic product (GDP) would fall by $1.6 trillion.
Yes, trillion, with a "T."
Republicans aren't backing President Obama's approach to immigration. But it doesn't negate the need for a solution.
In November 2014, Obama announced a sweeping deportation relief program that would have given an estimated 5 million the chance to live and work in the U.S. legally.
Republicans widely opposed the move and have kept it from moving forward through legal challenges. In the latest setback to the program, a federal appeals court that the program overstepped the president's authority. Now Obama wants the Supreme Court to hear the case.
Most Republican candidates for president oppose Obama's plan. But that doesn't mean there isn't room for compromise.
So far, talk about bigger border walls has dominated the Republican primary, but it doesn't have to remain that way.
After Kasich's comments, former Florida Gov. Jeb Bush added his thoughts, saying that deporting millions of people is "not possible" and "not embracing American values."
"It would tear communities apart," Bush said. "And it would send a signal that we're not the kind of country that I know America is."