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

They had never seen one before. Inside an enormous room that looks like a giant, bubbling kitchen, Wonka takes them to a machine that is dropping large green marble-sized candies out of it. He says they are Everlasting Gobstoppers, completely new. The gobstoppers can be sucked forever, and will never get any smaller. Violet says it is like gum, but Wonka reminds her that gum is for chewing, and chewing one of these would break your teeth.