Washington ? Doug Hardy was barely inside the door of the National Air and Space Museum when he made up his first “fact.”

On a sunny morning a few days before Father’s Day, Hardy and his son Andrei were huddled under the Mercury capsule. Like countless dads before him, he was explaining rocket science to his boy, in this case how the mottled heat shield protected John Glenn from a fiery death as the craft plunged through the atmosphere.

Then Andrei, 12, asked: What are these dark disks made of?

Hardy answered confidently – even though he didn’t have a clue.

“Steel,” he said.

(The shield is actually made from a plastic-fiberglass composite, said Michael Neufeld, chairman of the museum’s space history division.)

If it didn’t occur to Hardy to say, “I don’t know,” he’s not alone. The phenomenon of the “know-it-all dad” is a familiar one to the docents, curators and keepers of America’s museums and zoos.

“Just about every time I’m on the floor, I hear a father say something incorrect to his kids,” said Bobbe Dyke, who has been a docent and tour guide at Air and Space for 31 years. “You can’t butt in and correct them in front of the kids. You just have to cringe.”

Asked about the exchange a few minutes later, Hardy, a Boston-based writer, good-naturedly admitted his lack of metallurgical expertise.

“Now that I think about it, I guess I make up stuff all the time,” he said.

Only a few days earlier, at the Vietnam Veterans Memorial, Andrei had asked how bronze statues were made. Hardy finessed an explanation based on half-remembered notions of wax molds and plaster.

“It was a total BS moment,” Hardy said. “But you’ve got to be the guy who has the answers, right? It’s a habit. What should I say, that I’m 51 years old and I used to know this 20 years ago? That’s not much of an answer.”

Standing in the museum’s entrance hall, Dyke can attach overheard blarney to just about every icon on display:

¢ Friendship 7: John Glenn flew the little capsule to the moon. (He was the first American to orbit the Earth.)

¢ Sputnik: The Russian satellite carried a dog into space. (The sphere – the one at the museum is a replica – is less than 23 inches in diameter.)

¢ The Bell X-1: The sound-barrier-busting aircraft was built without landing gear to make it faster. (The wheels are just retracted.)

Most of the know-it-all dads do start out with a baseline of accurate data, said Beth Wilson, a museum education specialist. They get tripped up when they try to go too deep.

“They have some working knowledge of a subject without fully grasping the details,” she said. “Sometimes we say that a little PBS is a dangerous thing.”