Image copyright Reuters Image caption A US marine carries the nuclear football onto a helicopter in November

The president has warned North Korea on Twitter that he has a "nuclear button" - but does he?

Firing a nuclear weapon is - unsurprisingly - more complex than changing the TV channel.

More surprisingly, it involves biscuits and footballs.

While the term "nuclear button" is a well-known shorthand, the answer is clear.

Donald Trump does not, literally, have a nuclear button.

So what does he have?

On 20 January last year, a military aide carrying a leather briefcase went to Donald Trump's inauguration with President Obama.

After Mr Trump took the oath of office, the aide - and the briefcase - moved to Mr Trump's side.

The briefcase is known as the "nuclear football". The football is needed to fire US nuclear weapons and - in theory - never leaves the president's side.

In August, an expert told CNN that, when Mr Trump plays golf, the football follows him round the course in a buggy.

What's in the football?

If the public ever peeped inside the nuclear football, they might be disappointed.

There is no button. And there is no clock, ticking down to Armageddon.

Instead, there are communication tools and books with prepared war plans.

The plans are designed for quick decision-making.

In 1980, Bill Gulley - a former director of the White House Military Office - said retaliatory options boil down to "rare, medium, or well done".

So what is the biscuit?

The "biscuit" is a card containing codes, which is supposed to be carried by the president at all times.

It is separate to the football.

If the president were to order a strike, he would use the codes to identify himself to the military.

After taking office, ABC News asked Mr Trump how it felt to receive "the biscuit".

"When they explain what it represents, and the kind of destruction that you're talking about, it is a very sobering moment," he said.

"It's very, very scary, in a sense."

Media playback is unsupported on your device Media caption Which countries have nuclear weapons?

A former military aide to Bill Clinton, Robert "Buzz" Patterson, claimed Mr Clinton lost the codes while president.

Mr Patterson said Mr Clinton would keep the biscuit in his trouser pocket - attached to his credit cards with a rubber band.

On the morning the Lewinsky scandal broke, Mr Clinton admitted he hadn't seen the codes for some time, according to Mr Patterson.

Another top-ranking officer - General Hugh Shelton - also claimed Mr Clinton lost the biscuit for "months".

US nuclear codes 'went missing'

How does the president launch a nuclear attack?

Only the president can launch nuclear strikes.

Once he has identified himself, he passes his order to the Chairman of the Joint Chiefs of Staff. The chairman is the highest-ranking US military officer.

The order then goes to the US Strategic Command HQ in Offutt Airbase, Nebraska.

They then pass it to "on the ground" teams (although these may be at sea, or under water).

The order to fire is transmitted via codes - which must match codes locked in the launch team's safes.

Can the president be disobeyed?

The president is commander-in-chief of the US military. In short, what he says, goes.

But there is - possibly - some leeway.

In November, for the first time in 40 years, Congress examined the president's authority to launch a nuclear attack.

One of the experts was C Robert Kehler, commander of the US Strategic Command from 2011-13.

He told the committee that, as trained, he would follow the president's nuclear order - but only if it were legal.

Under certain circumstances, he explained: "I would have said 'I'm not ready to proceed.'"

One senator asked: "Then what happens?" Mr Kehler admitted: "I don't know."

The committee laughed in response.

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Senate questions Trump's nuclear powers