<human>:  Why is a burger called a hamburger?
<bot>: The burger was first served in the US in the 1930s. It is named after the town of Hamburg, Germany. Hamburgers were invented by a man named Louis Lassen. Lassen created the first hamburger by putting ground beef between two pieces of bread, and then he added ketchup, mustard, pickles, and onions to make a sandwich.