What did Merlin write the message on?

The correct answer to this question is Seashell. Merlin is a fictional character from the book Summer of the Sea Serpent. The book was written by Mary Pope Osborne. It was published on March 9, 2004, and characterized as a fantasy fiction book.

Summer of the Sea Serpent is apart of the series Magic Tree House Merlin Mission Series, and it is the third book in that series. It was preceded by Haunted Castle on Hallows Eve and followed by Winter of the Ice Wizard. In these series, Merlin is a magician who makes Jack and Annie go on missions.