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Karl Ehrhardt (November 26, 1924 – February 5, 2008) was one of the New York Mets' most visible fans and an icon at Shea Stadium from its opening in 1964 through 1981. Known as the "Sign Man," Ehrhardt held up 20-by-26-inch black cardboard signs with sayings in big white (sometimes orange) upper-cased paper characters that reflected the Mets' performance on the field, and echoed the fans' sentiments off of it. He usually brought a portfolio holding about sixty of his 1,200 signs to the stadium, each of them with color-coded file tabs for different situations. He was always positioned in the field-level box seats on the third base side, wearing a black derby with a royal-blue-and-orange band around the bottom of the crown and the primary Mets logo on the front. Ehrhardt wasn't afraid to criticize the team's front office, once holding up a sign labelling Shea Stadium as "GRANT'S TOMB", referring to the team's miserable play and M. Donald Grant, the team's chairman of the board.
Karl Kurt Ehrhardt was born in Unterweissbach, Germany. He emigrated with his family to the United States at the age of six, settling in Brooklyn, New York where he grew up rooting for the hometown Dodgers. During World War II he served in the U.S. Army as a translator in a prisoner-of-war camp holding captured German soldiers. Following the war, he graduated from Pratt Institute with a degree in design art. He would later work as a commercial artist designing advertisements for American Home Foods. He was a resident of the Glen Oaks section of Queens in New York City.
Ehrhardt was once the subject of a feature by Heywood Hale Broun for a Saturday installment of the CBS Evening News in April 1969. This would be reshown on ESPN Classic in 2003 as part of an episode of Woodie's World about Broun's coverage of the Miracle Mets.
A sampling of his messages 
- "AMAZIN'!" - Based on the team's nickname which was first coined by Casey Stengel, the franchise's original manager.
- "MET POWER!" - Which he proudly displayed after Tommie Agee hit his leadoff home run in Game 3 of the 1969 World Series
- "JUST GREAT!"
- "BACK TO YOUR NEST, BIRD!" - Which appeared during the 1969 World Series against the Baltimore Orioles. This sign is seen in the highlight film during Game 5.
- "CAN YOU BELIEVE IT?"
- "CURSES! FOILED AGAIN"
- "AAUGHH" - Inspired by the Peanuts cartoon strip; it was used for whenever the Mets lost a game.
- "TOOTHLESS CUBS JUST A LOTTA LIP" - Which he displayed during Mets games against the fading Cubs in 1969, referring to Leo "The Lip" Durocher.
- "LOOK MA, NO HANDS" - Was shown when a slow grounder defied the grip of Mets' shortstop Frank Taveras at a summer Mets game in 1979.
- "JOSE, CAN YOU SEE?" - Presented when Cleveland Indians' outfielder José Cardenal struck out at a 1968-1969 Mets game.
- "IT'S ALIVE! IT'S ALIVE!" - For weak hitters who rarely reached base. A head shot of Frankenstein's monster was to the left of the letters on the sign.
- "SIT DOWN, YA BUM!" - For whenever a Dodgers fan was caught poking fun at the Mets at a Mets' game; back then, the Los Angeles Dodgers were referred to the "Los Angeles Bums".
- "LEAVE IT TO SEAVER" - Inspired by famous 1950s-1960s sitcom show, Leave It to Beaver; the sign was used for whenever Mets' pitcher Tom Seaver was called up to pitch.
- "A" and "G" - Which he held in each hand, raising and lowering each, to punctuate the crowd's chanting of center fielder Tommie Agee's name, after his second game-saving catch in Game 3 of the 1969 World Series.
- "DO YOUR THING HEYWOOD" - Flashed at Heywood Hale Broun at the end of his 1969 feature about Ehrhardt on the CBS Evening News.
- "BELIEVE IN MIRACLES?" - Flashed during the decisive Game 5 of the 1969 World Series.
- "BYE, BYE, BIRDIES" - Flashed during the same game.
- "THERE ARE NO WORDS" - The sign that Ehrhardt held up when the Mets' left fielder Cleon Jones caught the final out to clinch the team's first World Series Championship. This was his most famous creation, seen in the Series highlight film.
- "THEY SAID IT COULDN'T BE DONE" - Held high from a convertible, as Ehrhardt rode with the Mets' victory parade in the Canyon of Heroes in lower Manhattan.
- "NAILED BY THE (picture of a hammer)" - Held up after a home run was hit by slugging first baseman John Milner, whose nickname was "The Hammer".
- "YOU'RE FIRED!" - Held up during Game Three of the 1973 World Series when the Oakland Athletics committed an error. The sign referred to A's owner Charlie Finley's attempt to have infielder Mike Andrews removed from the team after a pair of difficult Game Two errors in the twelfth inning helped the Mets win the game.
- "KONG!" - For Dave Kingman's first regular season home run at home as a Met, helping to tag Kingman with the nickname King Kong.
- "THE KING OF SWING" - Another tribute to Kingman, drawing on the nickname given jazz legend Benny Goodman.
- "THE SIGNMAN LIVES!" - Used on his return to Shea Stadium at a game against the Los Angeles Dodgers in August, 2002 to help celebrate the Mets' 40th anniversary.
- Mallozzi, Vincent M. "Recalling the Time of the Signs at Shea," The New York Times, Sunday, June 18, 2006.
- Hevesi, Dennis. "Karl Ehrhardt, 83, Sign Man and Shea Stadium Fixture," The New York Times, Saturday, February 9, 2008.
- Nichols, Adam. "Famed Mets 'Sign Man,' 83, signs off," New York Daily News, Sunday, February 10, 2008.