“For me, nuclear, the power, the devastation, is very important to me,” Trump replied.

But “nuclear,” “the power” and “the devastation” aren’t the three legs of the U.S.’s nuclear triad.

So what are the components of the nuclear triad?

The nuclear triad refers to the three ways the U.S. is capable of firing nuclear weapons.

As Florida Sen. Marco Rubio explained during the debate following Trump’s mishmash of a response: “The triad is the ability of the United States to conduct nuclear attacks using airplanes, using missiles launched from silos from the ground and from our nuclear subs.”

To add a little more specificity, the planes are heavy bombers; the silos house intercontinental ballistic missiles and the submarines also use ballistic missiles to deliver a nuclear payload.

Rubio, who avoided attacking Trump on Tuesday, didn’t directly call out Trump for blanking on the national security question. Instead, he directed his explanation to the “people at home” who likely “have not heard that terminology before.”

The Trump campaign didn’t respond to a request for comment.

So why does the U.S. need three ways of delivering nukes?

Rubio summed it up as: “All three of them are critical. It gives us the ability at deterrence.”

In more expansive terms, they’re all key components because they protect the U.S.’s ability to launch nuclear strikes should one or two of those capabilities be destroyed.

If the underground silos backfire and the planes capable of delivering nuclear weapons get destroyed, the U.S. would still have stealthy nuclear submarines to deliver crippling strike.

The U.S. and Russia are the only two nuclear powers in the world to have triad capabilities, and both countries are eager to maintain that edge going forward.

Makes sense. So what’s so pressing that this had to be included in the debate?

All three components of the nuclear triad are aging and the next president is going to have to address that issue.

America’s nuclear submarines are all more than 30 years old and its most dominant bomber jets remain the 60-year-old B-52s. The Pentagon has also called for upgrading the U.S. arsenal of ICBMs, or intercontinental ballistic missiles.

The Pentagon has estimated that it will need to spend as much as $18 billion per year over the next 15 years — for a grand total of $270 billion — to modernize the nuclear triad.

Amid budget cuts on Capitol Hill, it’s struggled to come up with the funding to get that job done.