There are plenty of rumors about President Obama. Some people think he wasn't born in America; others are convinced he's the Antichrist. But the most resilient (and most widely-believed) rumor is that he's Muslim. A new poll from the Pew Forum on Religion and Public Life found that 18 percent of Americans surveyed believe that Obama's faith is Islam—a number that has increased by 7 percent since March 2009.

How do we know Barack Obama is not Muslim? For those who know something about Islam, there are a few clues.

First, Obama has denied being Muslim. This by itself violates Islamic teachings, which require shahadah, or the profession of faith ("There is no God but Allah, and Mohammed is His prophet"). He has also never been seen performing salat, the ritual prayer to Mecca, five times per day — something that would have been noticed during his nearly 600 days in office. Furthermore, for many years Obama attended Trinity United Church of Christ — no-no for devout Muslims.

Obama is also not observing Ramadan, the ninth and holiest month of the Islamic calendar. Muslims must fast each day of Ramadan from sunrise to sunset; it began Aug. 11 and will conclude around Sept. 10. Since Obama has been seen eating during daylight hours, he is not observing Ramadan and is therefore not following Islam.

Then there's the fact that Obama has never made a pilgrimage to Mecca, as required by Islam (though it need only be done once in a lifetime, so there's still time in case he converts). In all, Obama is not exhibiting any behavior that distinguishes Muslims from non-Muslims.

So given the complete lack of evidence that Obama is Muslim (and the overwhelming evidence that he is not), why are so many people misinformed about such a basic issue?

There seem to be at least two reasons. Joshua DuBois, director of the White House Office of Faith-Based and Neighborhood Partnerships, blamed misinformation campaigns and "an effort by some to regularly malign and distort [Obama's] faith." The other element is a news media that has misinformed its audience; among those who believe that Obama is Muslim, 60 percent learned about his religion from the media. And though Obama prays daily and meets with Christian spiritual advisors, he has chosen to keep his faith a largely private matter.

Benjamin Radford is managing editor of the Skeptical Inquirer science magazine. His new book is Scientific Paranormal Investigation; this and his other books and projects can be found on his website. His Bad Science column appears regularly on LiveScience.