Why were the children and their parents so surprised when a piece of gum appeared ?

Finally they make it over to another machine, and Wonka says he is incredibly proud of this invention. He presses a button on the side of the machine and it begins to whir and whiz. At last, out of the machine comes a little grey strip of something. They are confused for a moment, but Violet is the first to realize that this is a stick of chewing gum.