Image copyright EPA Image caption Children from around the US called to speak to the president and his wife on Christmas Eve

There are some questions no-one should ever ask.

"What does this red button do?"

"What do people say about me?"

"Do you believe in Santa Claus?"

Now we all know Santa Claus exists. But for some reason, the US president threw doubt on this when speaking to one child on Christmas Eve.

The scene was the State Dining Room in the White House.

Donald Trump and his wife Melania were taking calls from American children as the couple sat under two gargantuan Christmas trees.

All the children who called in had done so in the hope of getting through to Norad, the government agency that tracks Santa's movements around the world at Christmas (that is still operating despite a partial government shutdown).

Some of those calls were patched through to Mr Trump and his wife, and thanks to pool reporter Kevin Diaz, we know some of what the president said.

This is how Diaz reported the exchange:

Trump (in booming voice) to a kid named Collman: "Hello, is this Collman? Merry Christmas. How are you? How old are you?.... Are you doing well in school? Are you still a believer in Santa?"

Footage of the incident circulating on social media shows Mr Trump then telling the child: "Because at seven, it's marginal, right?"

"Collman", it now emerges, is a seven-year-old girl from South Carolina named Collman Lloyd, who told the Post and Courier newspaper she didn't know what "marginal" meant.

It is not clear why Mr Trump asked this particular question, because, of course, Santa's existence is beyond dispute.

These photos taken around the world over the past few hours provide overwhelming evidence he exists:

Image copyright Reuters Image caption Here he is in Ciudad Juarez, Mexico...

Image copyright Getty Images Image caption ...and here he is in Disneyland...

Image copyright Reuters Image caption ...and here he is in Espoo, Finland

The rest of the calls passed without incident, with warm wishes from both Trumps to those who called in.

"I hope your dreams come true," Melania told one caller.