Anti-vaccine activist Robert F. Kennedy, Jr. and actor Robert De Niro held a press conference on Wednesday where they offered $100,000 to anyone who could prove vaccines are safe.

In a statement, they offered the money "to the first journalist, or other individual, who can point to a peer-reviewed scientific study demonstrating that thimerosal is safe in the amounts contained in vaccines currently being administered to American children and pregnant women."

There are a number of problems with this challenge.

Innumerable scientific studies have shown that low doses of thimerosal found in vaccines are safe. The CDC lists nine such studies — theoretically, any of these could be used to collect the $100,000.

However, as science blogger David Gorski pointed out, the challenge is fraught with conditions. The "winning" study requires confirmation from a panel of "independent judges" that Kennedy and DeNiro will select, but haven't named. It also requires payment of a nonrefundable $50 fee to Kennedy's foundation, along with $200 per hour to the judges.

Such complicated terms and demands for money are a central part of challenges like this, which are common in psuedoscience. They almost always exist not to ensure scientific rigor, but to bring publicity to those who issue them, offering dubious amounts of money to either "disprove" things that either have already been demonstrated, or prove things that can't be proven. They are offered with the knowledge no one, by design, will win.

One such challenge was from Joseph Mastropaolo, a California kinesiologist who has had a long-standing "offer" of a $10,000 prize to anyone who can scientifically disprove the Genesis creation story in a legal trial — where he chose the judge. Mastropaolo required that the challenger put up $10,000 of their own money, with the winner netting both prizes.

Several biologists have attempted to take up the "Life Sciences Prize Challenge," but none could work out an arrangement to actually participate, further suggesting the challenge is in fact just a stunt.

A more lucrative, equally unwinnable "challenge" was offered by creationist Kent Hovind, known as "Dr. Dino" for founding an amusement park devoted to showing how dinosaurs allegedly interacted with early man.

In 2001, Hovind announced he was putting $250,000 on the line for “anyone who can give any empirical evidence (scientific proof) for evolution.”

In the next few years, Hovind continuously dodged specifics on how the proof would be judged, and came up with excuse after excuse as to why the responses he did get weren’t valid. Finally, in 2006, Hovind was found guilty of 58 counts of tax evasion, and was sent to federal prison.

Then there’s the largesse of flamboyant Turkish creationist and conspiracy theorist Adnan Oktar, who offered ten trillion Turkish lira (around $7 trillion) for a single transitional fossil that proves any organism has ever evolved — the so-called "missing link."

While numerous transitional fossils have been discovered and corroborated, Oktar's vast sum of money remains unclaimed, and he's moved on to creating what The Daily Beast describes as an "Islamic sex cult."

Beyond creationism, large amounts of money await anyone who can prove any number of unprovable theories.

• There are numerous prizes available for anyone who can create a free energy machine, which is nobody, because inexhaustible energy violates the laws of physics.
• Documentary filmmaker James Fox made an offer of $100,000 for definitive proof of the existence of aliens in the form of "a photograph, video or film footage or debris from an alleged crash site.” 
• Bio-ethicist Art Caplan put up $10,000 for evidence of brain damage from the HPV vaccine Gardasil, a response to former Republican Congresswoman Michelle Bachmann’s claim that the vaccine caused "mental retardation” in one of her constituents.
• In response, anti-vaccine group SAFE Vax Inc. made its own offer of $10,000 for proof Gardasil had prevented a single case of HPV. Neither was ever claimed. 

Even if Kennedy and De Niro's challenge is accepted — and won — they always have the option of trying to weasel out of paying the money. But this can carry tremendous legal risk. 

in 1980, Holocaust denial group the Institute for Historical Review offered $50,000 for proof that any Jews were gassed at Auschwitz. Camp survivor Mel Mermelstein submitted a notarized letter detailing how he watched familes led to gas chambers and killed. Even so, the IHR refused to pay, and Mermelstein sued the group.

United States Holocaust Memorial Museum - ushmm.org

The subsequent trial found that the veracity of the Holocaust “was not reasonably subject to dispute," and Mermelstein was awarded the $50,000, plus damages. 

More recently, a German anti-vaccine doctor who offered 100,000 euros ($106,000) for proof that measles is an actual virus was ordered by a court to pay the award to a biologist who did just that using peer-reviewed research.

These cases suggest that if Kenney and De Niro are unwilling to pay up, a winner might be found in court.