Question: Who invented the hamburger?
About the article "Hamburger", section "Charlie Nagreen".

Candidate passages:
The hamburger bun was invented in 1916 by a fry cook named Walter Anderson, who co-founded White Castle in 1921.  
In August 2006 Coondog co-hosted the The National Hamburger Festival in Akron, Ohio.
The festival was held to honor local food innovators, Frank and Charles Menches, who claimed to have invented the hamburger in 1885.
The festival included a hamburger eating contest, which O'Karma won, by eating 4 and 1/2 double decker hamburgers in 8 minutes.
In 2002 he left the IFOCE after conflicts occurred with its owner, George Shea.
He disagreed with IFOCE management regarding several issues, including concerns about the health and pay of competitive eaters.
He later joined forces with a smaller Competitive Eating league, the Association of Independent Competitive Eaters (AICE), which was established by his friend eater Arnie "Chowhound" Chapman.
He now serves as the Director of Operations for the AICE. 
The exact origin of the hamburger is unknown and may never be known with any certainty.
Most historians believe that it was invented by a cook who placed a Hamburg steak between two slices of bread in a small town in Texas, others credit the founder of White Castle for developing the "Hamburger Sandwich" but since records are scarce from that time we will never know for sure.
However, all claims made by the potential inventors of the hamburger occurred between 1885 and 1904, focusing all attention of its creation onto these two decades.
Despite the various stories about the origins of the hamburger, there are common elements in all of the narratives, most notably that the hamburger was born as a food associated with major events such as amusement parks, fairs, conferences, and festivals.
All the hypotheses also share the presence of street vendors.
One of the possible fathers of the hamburger is Charlie Nagreen (1870–1951) of Seymour, Wisconsin, who at the age of 15 sold Hamburg steaks from a street stall at the annual Outagamie County Fair.
Nagreen said he began by selling Hamburg steaks, but these did not have much success because people wanted to freely move around the festival without the need to eat them at his stand.
In response to this, in 1885 Nagreen decided to flatten the hamburger steak and insert it between two slices of bread, so that the public could move freely from booth to booth while eating his sandwich, an innovation that was well received by his customers.
This became known as the "Hamburger Charlie", and Nagreen's creation was sold at the festival until his death in 1951.
To this day, his accomplishment is celebrated annually with a "Burger Fest" in his honor in his hometown of Seymour.
Another alleged inventor of the hamburger is the cook Fletcher Davis (better known as "Old Dave"), who claimed to have had the idea of putting ground beef between two pieces of Texas Toast when one of his customers was in too big of a hurry to sit down for a meal.
The customer walked away with his Hamburger Steak and seems so content that Old Dave offered his new carryout meal as a staple menu item at the end of 1880 in Athens, Texas.
Fletcher had a stall with his wife at the St. Louis World's Fair of 1904.
Texan journalist Frank X. Tolbert mentions a salesman named Fletcher Davis who served hamburgers in a café at 115 Tyler Street in Athens during the late 1880s.
The locals claim that Davis was selling beef sandwiches during that time, without having a clear name for his invention.
During the 1980s, the Dairy Queen ice cream chain filmed a documentary about the birthplace of the hamburger featuring Davis' story.
The story of "Old Dave's Hamburger Sandwich" is also found mentioned in Ronald McDonald's book "The Complete Hamburger" a collbrative effort with Richard McDonald the Founder of McDonald'sIn the same year that Charlie Nagreen claimed to have developed his "Hamburger Charlie", the brothers and street cooks Frank and Charles Menches of Akron, Ohio claimed to have sold a ground beef sandwich at the Erie County Fair.
According to the brothers, the name of the hamburger derived from the town Hamburg, New York in Erie County, the first place in which it had been sold.
This statement has been very poorly documented or substantiated, and in the case of an oral tradition, it is not without its contradictions.
Its famous description of the secret ingredients used in the recipes, such as coffee or brown sugar, is primitive.
Another version of the creation of the hamburger is that of German cook Otto Kuasw, who created a very popular sailors' sandwich made of a fillet of beef patty fried in butter, served with a fried egg, between two toasted buns in 1891, at a post in Hamburg, Germany.
The sandwich was called the "Deutsches Beefsteak", German for "German beefsteak".
Many of the sailors traveling on ships between Hamburg and New York requested a similar "Hamburg style" sandwich at American steakhouses.
There are many additional claims as to the origin of the hambuger.
Some enthusiasts claim the hamburger could have been created in New Haven, Connecticut in 1895, but this claim has no supporting evidence to back the opinion.
A local newspaper claims that Louis Lassen, a Danish immigrant who arrived in America in 1880, sold butter and eggs as a street vendor.
In 1974, in an interview with "The New York Times", he recounted how he created a hamburger steak sandwich with small strips of beef for a restaurant known as Louis' Lunch.
Lassen's family claims 1900 as the year of his invention.
Revealed in a public confrontation between the grandson of Louis, Kenneth Davis Lassen Fletch, and his uncle, documentation signed under notary in 1900 demonstrates the development of the hamburger, as it stresses the difference between a "hamburger steak" and a "hamburger sandwich".
Louis' Lunch flame broils the hamburgers in the original vertical cast iron gas stoves manufactured by the Bridge and Beach, Co., St. Louis, Missouri in 1898.
The stoves use hinged steel wire gridirons to hold the hamburgers in place while they cook simultaneously on both sides.
The gridirons were made by Luigi Pieragostini and patented in 1938. 
The show centers on the Belcher family, who run a hamburger restaurant on Ocean Avenue. 
Viktor Hamburger (July 9, 1900 – June 12, 2001) was a German professor and embryologist.
Hamburger lectured, among others, Nobel Prize-winning neurologist Rita Levi-Montalcini, who identified nerve growth factor along with Hamburger when they collaborated.
Hamburger began to work at Washington University in St. Louis in 1935; he retired from his professor position in 1969 and continued researching until the 1980s.