PURCHASE, N.Y. — Why do people sing "Take Me Out to the Ball Game" when they're already AT the game?

The answer to that question, along with anecdotes involving Yogi Berra, the Marx Brothers, "The X-Files" and many more can be found in a new book, "Baseball's Greatest Hit," celebrating the song's 100th anniversary.

Its rich history might be a revelation to the millions of Americans who will start singing "Take Me Out" this week as major league teams start their 2008 seasons.

For example, few realize that they're only singing the refrain — the full song is about a young woman. Its two verses tell the story of baseball-mad Katie Casey, whose beau invites her to the theater. She says "No, I'll tell you what you can do: Take me out to the ball game" and so on. In the second verse, she yells at the umpire.

"The refrain, as great as it is, is fairly light," says Tim Wiles, director of research at the library of the National Baseball Hall of Fame in Cooperstown and a co-author of the book. "But when you flesh it out with the full story, there's a lot there."

Another surprise is that the song became a hit long before it was sung at baseball games. Written in 1908 by composer Albert von Tizler and lyricist Jack Norworth, "Take Me Out" was sung during reel changes in movie theaters to promote sales of the sheet music.

"Americans were getting together and singing 'Take Me Out to the Ball Game' in movie theaters decades before they did it in ballparks," says co-author Bob Thompson, associate dean of the Conservatory of Music at Purchase College.

Thompson has Abbott and Costello's "Who's on First?" routine on his iPod. Wiles dresses up in a period baseball uniform to recite "Casey at the Bat" to groups ranging from Hall of Fame inductees to school groups. And the other co-author, sports marketer Andy Strasberg, has a doorbell on his San Diego home that plays, you guessed it, "Take Me Out to the Ball Game."

Their apparent obsession with baseball did not prevent a skeptical approach to some of the lore about the song. The heavily footnoted book casts considerable doubt on Norworth's account that he wrote the words during a New York subway ride. And the authors have dug up evidence that counters the story that the songwriters never went to a baseball game.

The history of the song, of course, is intertwined with the history of the seventh-inning stretch, when the song traditionally is sung at major league stadiums.

But the authors insist that tradition didn't reach full flower before the mid-1970s, when White Sox owner Bill Veeck and announcer Harry Caray turned it into a happening at Comiskey Park in Chicago. When Caray moved in 1981 to the Cubs and their national cable TV audience, a nationwide ritual was born.

Strasberg said the tune was played at some ballparks before games, and made intermittent appearances during the stretch, but was not an established seventh-inning ritual in the major leagues before Caray.

The authors' research also has sprinkled the heavily illustrated book with lists including:

— Hundreds of baseball songs going back to 1858's "The Baseball Polka." Thompson says "Take me Out to the Ball Game" wasn't even the first baseball hit; that honor goes to "Slide, Kelly, Slide" (1892).

— A discography of some recordings of the song, including artists from Harpo Marx to the Ink Spots to Bob Dylan. Harpo played the song on his harp during a 1955 appearance on TV's "I Love Lucy." The song also appears in the Marx Brothers' "A Night at the Opera" when the orchestra's music gets mixed up. The authors say the song has been heard on some 1,200 movies and TV shows, from "Arsenic and Old Lace" to "The X-Files."

Among the efforts never recorded was one by Berra, Phil Rizzuto, Whitey Ford and Enos Slaughter singing the song in Austria with the Vienna Boys Choir, the book says. There are, however, recordings of the song in Japanese and Yiddish.

— A timeline with significant events in the song's history, including Major League Baseball's attempt to replace "Take Me Out" by commissioning a new song in 1969. The result was "Baseball is More Than a Game," and the authors say, "We've never heard it, have you?"

The book includes a foreword by Carly Simon, who says "Take Me Out" is tough to sing because "there's nowhere to breathe."

And it includes an authors' dedication: "To our moms, who gave us music, and our dads, who gave us baseball."