"Oh, of course, you wouldn't know - Chocolate Frogs have cards, inside them, you know, to collect - famous witches and wizards. I've got about five hundred, but I haven't got Agrippa or Ptolemy."
"I don't think there is a Ptolemy card," said Ron in frustration. "I have yet to meet anybody who has had one or even seen one."
There were several mutters of agreement while students asked their friends if they had seen one or not.
"Ah," called Professor Dumbledore. "But Mr. Weasley, there is in fact a Ptolemy card. They are just the rarest of the chocolate frog cards. There is only one in ten million cards made. It was done this way so that people will keep buying the frogs in the hope of getting it. I, in fact, do have one myself but am unwilling to part with it. It took me thirty years to find it."
"One in ten million," Ron mumbled in a daze. "Thirty years? That's a lot of frogs to eat. Oh well, better start eating then."
Harry and Hermione snickered at Ron's new goal.