Bill thought there were three pieces of pizza left in the box, but he was wrong. Richard and Stephanie are leaving the pizza shop with a box of pizza. George believes that there are three pieces of pizza in the box. Suppose that his belief is true. Then by our definition, we would have to say that he knows that there are three pieces of pizza in the box.
Most people say no.
Stephanie says that there are eight pieces of pizza in the box. She and Richard just bought the pizza five minutes ago and she knows that the pizza shop always cuts pizzas into eight slices. But as she is coming out of the bathroom, she sees Richard putting the last bite of a piece of pizza in his mouth. A belief is impossible to doubt if it is impossible to imagine a way in which my evidence for believing it could be undermined.