LAS VEGAS -- Toward the end of Tuesday night's Republican presidential debate here, front-runner Donald Trump was asked about the U.S. nuclear triad, but seemed not to know what it was.

After the billionaire fumbled, Florida Sen. Marco Rubio picked up the ball to explain the concept to "people at home" who might be unfamiliar with the term -- in addition to Trump. Here's the full transcript of the exchange:

"What's your priority among our nuclear triad?"

Trump: "Well, first of all, I think we need somebody absolutely that we can trust, who is totally responsible; who really knows what he or she is doing. That is so powerful and so important. And one of the things that I'm frankly most proud of is that in 2003, 2004, I was totally against going into Iraq because you're going to destabilize the Middle East. I called it. I called it very strongly. And it was very important.

"But we have to be extremely vigilant and extremely careful when it comes to nuclear. Nuclear changes the whole ball game. Frankly, I would have said get out of Syria; get out -- if we didn't have the power of weaponry today. The power is so massive that we can't just leave areas that 50 years ago or 75 years ago we wouldn't care. It was hand-to-hand combat.

"The biggest problem this world has today is not President Obama with global warming, which is inconceivable, this is what he's saying. The biggest problem we have is nuclear -- nuclear proliferation and having some maniac, having some madman go out and get a nuclear weapon. That's in my opinion, that is the single biggest problem that our country faces right now.

Hewitt: "Of the three legs of the triad, though, do you have a priority? I want to go to Senator Rubio after that and ask him."

Trump: "I think -- I think, for me, nuclear is just the power, the devastation is very important to me."

Hewitt: "Senator Rubio, do you have a response?"

Rubio: "I do. First, let's explain to people at home who the triad -- what the triad is. Maybe a lot of people haven't heard that terminology before. The triad is our ability of the United States to conduct nuclear attacks using airplanes, using missiles launched from silos or from the ground, and also from our nuclear subs' ability to attack. And it's important -- all three of them are critical. It gives us the ability at deterrence.

"Now, some have become more critical than others; for example, the submarines. And that's the Ohio Class submarine that needs to be modernized. The air component also needs to be modernized. The B-52, as someone earlier pointed out, is an outdated model that was flown by the grandparents of people that are flying it now. And we need a serious modernization program as well on our silo-launched missiles. All three are critical for the defense of the country."

We asked Jeb Bush campaign manager Danny Diaz in the spin room if Trump's triad answer would haunt him. Diaz replied:

"We've said for quite a while here that we don't believe that his kind of depth of knowledge on these issues, that coupled with kind of erratic kind of behavior and rhetoric, are suited for the highest office in the country."