Democratic debate: Protesters interrupt Joe Biden during his closing segment

Buttigieg says that the leader of the US needs to believe in public education.

"I believe in public education," he says.

"I think I'm the only person on this stage who has been a public school teacher," Warren says when asked if she is simply "jumping into bed" with teachers unions, as Yang said.

She reiterates her promise to make the secretary of Education a person who has been a public school teacher.

One of the biggest applause lines of the night just came after Warren reiterated her promise to cancel student loan debt for most Americans.

Harris calling for more diversity in the public education system, saying that black children who have black teachers while they are young are more likely to go to college.

Sanders says "we are the wealthiest country in the history of the world", but contrasts that with the problems in the US. He says that teachers in the US need to make at least $60,000 a year.

"We are going to cancel all student debt in this country. And we are going to do that by imposing a tax on Wall Street speculation," Sanders says.

When asked about resiliency, Biden was interrupted by protesters who derailed the debates for a moment.

It is unclear what they were yelling there in Houston.

Asked about resilience, Biden talks about the death of his wife Neilia and daughter Naomi in a car crash in 1972, and the death of his son Beau of brain cancer in 2015.

Cory Booker recounts taking on the "political machine" in Newark, New Jersey, in 2002 and having his tyres slashed and his phone tapped - and losing the election. But he won next time around. He quotes an old African saying: If you want to go fast, go alone - if you want to go far, go together.

Beto O'Rourke describes a shooting victim who is still trying to coach his soccer team from his hospital bed. He says the people of El Paso will set an example to the rest of the country.