"Didn't you tell me that UFO's had been around as far back as 1800?" asked Max.
"Oh yes," said Nina. "This one professor said it could be even farther back. Why?"
"Well, Sam Cartland said they had only been reported since 1947. That's only 51 years ago."
"Oh, oh," said Nina. "That part about seeing them way back came at the very end of the show. Do you suppose?"
Here's another clue: