“SAI!” Marly and Isla yelled.

“Down here,” Sai called from somewhere downstairs. “I think I found another puzzle.”

Marly and Isla glanced at one another, then charged down the stairs.

“Where?” Isla asked. “Where are you?”

“In here.”

Marly and Isla followed Sai’s voice into the living room and found him hunched over the Scrabble game.

“I don’t think this is a real game,” he said, staring down at the board. “See? None of the words go over the starting square.” He touched the pink square in the middle of the board.

“Also, there aren’t any racks of letters on the table,” Isla pointed out.

“Maybe the words are another message for us,” Marly said. She opened their notebook to a clean page and started copying down all the words on the board: first, dictionary, the, letter, in, three, scrabble, and words. Then she sat down on the overstuffed sofa and tried to rearrange the words into a sentence.

“First three two-letter words in the dictionary?” Marly said as Isla and Sai plopped down beside her. “No. We didn’t use the word Scrabble.”

Isla twirled her hair around her finger. “Well, there is such a thing as a Scrabble dictionary,” she said. “So it could say ‘First three two-letter words in the Scrabble dictionary.’ Is there a Scrabble dictionary in here?”

Sai hopped over to one of the bookshelves, then tilted his head as he scanned the books.

Isla checked the other shelf. “There is one!” she cried, pulling out a hardcover book. “I’m not surprised. Mr. Summerling always liked weird words. And there are lots of weird words in here. Like words that have a Q, but no U.”

“What?” Marly said. She’d never heard of a word that has a Q in it but no U.

“We don’t care about weird words right now,” Sai said, taking the book from Isla. “We care about the first three two-letter words.” He flipped back to the beginning of the book.

Marly studied her notebook. “You know? The message could also say ‘First two three-letter words in the Scrabble dictionary,’” she said, chewing on the end of her pencil.

“Wait, what was that?” Isla cried.

“What was what?” Marly lifted her head. She didn’t hear anything.

But Isla was clearly talking about something she’d seen in the dictionary. Not something she’d heard elsewhere in the house. Isla took the dictionary back and quickly turned the pages until she found what she was looking for.

“Look.” She held the book so Marly and Sai could see the small rectangle cut into the pages of the book. Tucked inside the rectangle was a silver key.

“Wow!” Marly’s eye opened wide. “That’s a great place to hide something!”

“This could be the very important key.” Sai grabbed it, accidentally knocking the book from Isla’s hands onto the floor. “I bet it unlocks that door at the end of the hall!” He bolted for the stairs.

“Wait, Sai,” Isla said, picking up the book. “The whole reason we went looking for a Scrabble dictionary was so we could find the first three two-letter words, remember?”

“Or the first two three-letter words,” Marly added. “That’s what the puzzle was really about. Finding those words. We need to find them and write them down.”

Sai groaned and dragged himself back. “I think the whole point of that puzzle was to find this key!”

“There could be two reasons for the puzzle,” Isla said. “To find the key and look up those words.”

Marly agreed. “We may have to enter these words into some other puzzle we haven’t found yet,” she said. “I think we should look them up before we go off exploring. It won’t take long.”

Sai exhaled impatiently while Isla turned to the beginning of the book. “Okay, Marly,” she said. “The first three two-letter words are—” She frowned. “I don’t know how to pronounce them, so I’ll just spell them. A-A, A-B, and A-D.”

Marly copied them into the notebook. “You weren’t kidding about weird words,” she muttered. “What does A-A even mean?”

“It’s some kind of lava,” Isla said, reading from the book. “Ready for the first two three-letter words?”

“Sure am.”

“A-A-H and A-A-L,” Isla said.

“More weird words,” Sai said as Marly wrote them down. Isla closed the dictionary and set it on the fireplace mantel.

Marly couldn’t disagree. But she was glad they’d taken the time to copy those words into their notebook. Mr. Summerling never gave them a clue without a reason.

“Now can we take the very important key and try it in the locked door upstairs?” Sai asked.

“Yep,” Marly said, tucking the notebook into her bag.

They dashed for the stairs. But before they were even halfway up, the doorbell rang.