We've been reading the Mrs. Piggle Wiggle books which were favorites of mine "when I was a girl." When we read together, we always check to see when books were written.
Me: "Oh my gosh! This book was written back when Grandma was a girl!"
Declan: "So when was that?"
Me: "It was in the 40s."
Declan: "So Grandma was born in 18. . ."
Me: "Nineteen! Grandma was born in the 19s!"
Hysterical laughter from all parties in the room.