A burger is a sandwich built around a cooked ground meat patty -- most often beef -- served on a bun with toppings and condiments. Ground beef became widely affordable in the 19th century, and by the early 20th century vendors across the U.S. were serving hamburger sandwiches; exactly who served the first one is disputed, with several vendors claiming the title, and the 1904 St. Louis World's Fair is often credited, probably wrongly, with inventing rather than simply popularizing it. Beyond the classic beef patty, burgers are now made with pork, poultry, or plant-based patties, and take regional forms such as the Japanese teriyaki burger or Indian masala burger; common toppings include lettuce, tomato, onion, pickles, cheese, and condiments like ketchup, mustard, and mayonnaise.