Who knew it was so hard to write a good dog-peeing-on-a-picnic-basket joke?

It must be nearly impossible, because for the past two decades, just about every movie bearing the National Lampoon imprint has been desperately – and unsuccessfully – trying to recapture the cinematic magic that was “Vacation.”

And “Animal House.”

And “European Vacation.” (“Hey, look, kids, there’s Big Ben! And there’s Parliament!”)

Despite the scores of movies the Lampoon has produced since the mid-’80s – most of which have gone straight to DVD – the company has yet to respin the kind of comedy gold that defined its early days.

You may be surprised, for example, to learn that they released a new movie just this fall.

“National Lampoon’s Adam and Eve,” a college-campus comedy, came out in a handful of theaters and disappeared immediately thereafter – hardly the trajectory of a classic comedy in the making.

And their next offering, “Pledge This” – set to open at some indeterminate time this year – seems doomed to follow suit. Yet another college romp, the movie boasts one asset: Paris Hilton in the role of the promiscuous sorority president.

But judging from the film’s trailer, she’s no Beverly D’Angelo.

So earlier this month, the foundering company announced a different tactic: they’re partnering with a newly-formed film studio, Half Shell Entertainment, to develop movies based on material from its magazine archives and website, nationallampoon.com.

Will this be enough to turn the Lampoon fortunes?

There are, after all, many comic gems to be found in the pages of the magazine, founded in 1970 by three grads from the Harvard Lampoon and featuring the writing of famed satirists like P.J. O’Rourke and “Spinal Tap” star Tony Hendra. The publication was the launching pad for the Lampoon’s other ventures, which included theater, radio and albums.

After all, “both ‘National Lampoon’s Animal House’ and the original ‘National Lampoon’s Vacation’ were spawned by original articles from the magazine,” current editor-in-chief Scott Rubin said in a statement.

One of Rubin’s Lampoon forebears is watching with interest.

“They’ve made a bunch of mistakes, but I think they’re learning,” says Matty Simmons, founding publisher of the magazine and producer of the classic Lampoon movies.

Simmons attributes the company’s early success to synchronicity as much as talent – assembling a team of brilliant comic minds at exactly the right place at the right time.

For example, a staged musical comedy he produced in 1972, “Lemmings,” featured three unknowns: John Belushi, Christopher Guest and Chevy Chase.

“We were a little spoiled,” he says modestly. “We started off from scratch and three months later, we were in the black.'”

Now, it seems just the opposite is true – whatever the Lampoon touches turns to, well, the stuff of which good bathroom jokes are made.

Simmons maintains the franchise can still turn the tide if it learns to recognize good writing and make good business decisions – of which “Pledge This” was not one.

“As soon as they told me it was Paris Hilton, I said, ‘I don’t want to hear about it,’ ” he says.

And Simmons should know.

” ‘Animal House,’ ” he says, “is still making a bundle of money, 26 years after it was released!”

sara.stewart@nypost.com