Midway through [movie id="390378"]"Iron Man 2,"[/movie] Tony Stark decides to build a particle accelerator in his basement. Why? Because he's frickin' Iron Man -- does he need a reason? For the nitpickers among you, let's just say that pulsing reactor in his chest is causing some problems, and Stark needs to create an entirely new element to save his life. Yup, the stakes are high.

So Stark is putting the finishing touches on the machine that smashes together subatomic particles when Agent Coulson, who works for the superhero-focused spy agency S.H.I.E.L.D., stops by and notices something strange: Captain America's stars-and-stripes-bedecked shield. Coulson wants to know how in the world Stark came into possession of the superhero's equipment. Stark seems blissfully aware of its true owner, choosing to use the shield to prop up his homemade accelerator.

It's a funny moment that speaks to the [article id="1638031"]burgeoning Marvel universe[/article] on the big screen. [article id="1634469"]"The First Avenger: Captain America"[/article] is in the works for a July 2011 release, followed the next year by "The Avengers," which will bring together the Captain, Stark and other Marvel characters.

But why did the shield pop up at all in "IM2"? Director Jon Favreau explained how the Easter egg came about in a recent MTV News interview.

"We introduced Captain America's shield briefly in one shot in the last film," he said. "An [Industrial Light and Magic] artist put it in there as a joke to us for our cineSync sessions, when we're approving visual effects.

"They got a laugh out of it, and I was like, 'Leave it in, that's pretty cool -- let's see if anybody sees it,' " he explained. "Of course, everybody saw it -- so now it really was in his room, so we had to figure out how to deal with the reality that the shield was in his workshop."

OK, so that's the how of the Captain's shield. When it came to the why, however, Favreau was reluctant to delve into detail, especially when it came to his conversations with directors of upcoming Marvel films.

"We've talked about it," he said. "Let's see what ['Captain America' director] Joe Johnston does."

But Favreau did drop us one hint about a possible connection between Stark and the Captain: that Stark's father, Howard, invented the super-soldier serum that turns Steve Rogers into the Captain.

"Part of the history of the Stark family is the super-soldier program," Favreau said.

Check out everything we've got on "Iron Man 2."

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