Conversation A

(Ian is holding a card over the light bulb.)

Sarah: Ian? Why are you holding that card over the light bulb? We already held it up to the light and couldn’t see anything.

David: Yeah, be careful. You might burn it!

Ian: I knew it! Guys, look at what’s on this card!

David: It’s a letter “H”.

Sarah: But why didn’t we see it before?

Ian: The letter was written on this card with invisible ink.

David: Ohhh! I’ve read about that in detective stories.