Ally noticed it Saturday morning. Sarah was missing her smile.

“What's wrong?” Ally asked, walking toward her sister.

“Nothing,” Sarah said.

“Want to play checkers?”

“Are you kidding?” Sarah went to her room and shut the door.

What's that about? Ally thought. Sarah always wants to play games. And she always smiles.

Ally decided to get to the bottom of the mystery. She ran to her room, grabbed a pencil and a notebook, and began making a list of suspects.

She wrote down everyone she remembered Sarah talking to since yesterday: Mom, Dad, Chris and Grandpa.

Ally knew that to get specific answers, she needed specific questions-questions beginning with who, what, when, where and why. The big question was obvious: Why wasn't Sarah smiling?

Ally hoped that by asking around, she would find enough pieces to figure out the puzzle. Then maybe Sarah would smile again.

With her list of questions in hand, Ally set out to interview each person. Mom was first. Ally spotted her playing the piano.

Hmm . . . piano lessons. Mom was always reminding everyone to practice. And when you forgot— goodbye, dessert! Maybe that was where Sarah's smile had gone.

“Hi, Mom,” Ally said, raising an eyebrow in suspicion. “When was the last time you saw Sarah smile?”

Mom stopped playing and wrinkled her forehead. “Sarah laughed last night after dinner when Grandpa's dentures fell out.”

Mmm . . . Ally mumbled. She jotted down the information and crossed Mom off the list.

Dad was suspect No. 2. Ally found him eating breakfast. It reminded her of last night's dessert—chocolate chip cookies. Sarah's favorite. Sarah always ate them slowly, dunking them in milk. Dad, on the other hand, gobbled them up, one after another. If you weren't quick, the cookies were gone.

“Hey, Dad,” Ally said. “Where did you last see Sarah smile?”

Dad scratched his chin. “Good question.”

“I thought so, too,” Ally said.

“Didn't Sarah laugh in the living room after dinner? Things got pretty noisy in there last night.”

“I see,” Ally nodded. She scribbled the information into her notebook.

Chris was next. Her brother was in the living room, watching cartoons. He was also sitting in the comfiest chair in the house. Very interesting, Ally thought. Sometimes there were arguments about who got to sit in that chair.

“What happened to Sarah last night here in the living room that made her stop laughing?” Ally asked.

Chris shrugged. “How should I know?”

Ally squinted and stood in front of the TV screen.

“Wasn't she playing cards?” Chris asked.

“Aha,” Ally said.

Ally looked at her notes. So far, Sarah's misery was still a mystery. But Ally felt she was getting closer to the answer. And she still had one more person to question: Grandpa.

Chris had said that Sarah played cards last night. Grandpa often played cards with the family. Was he at the bottom of the missing smile?

Ally bumped into him while crossing Chris off the list.

“Good morning, Ally,” Grandpa said. “What are you writing about?”

“I'm solving a mystery.”

Grandpa rubbed his hands together. “I love a good mystery.”

Ally stared into his eyes. “Who did Sarah play cards with last night?”

Grandpa's brown eyes opened wide. “Me!”

Ally jotted it down and closed her notebook.

“Wait a second,” Grandpa said. “I recall someone else was playing with us, too. Isn't that right?”

This detail seemed most important.

And familiar.

“Bingo,” Ally whispered. She opened her notebook and looked back up at Grandpa, her pencil ready to record. “And who was that?”

“Who was it?” Grandpa tapped his foot. “Sorry, Ally, but I don't remember.”

“No problem, Grandpa.”

“Later, Ally Gator!” Grandpa left the room.

Ally chewed on the end of her pencil, thinking things through. Someone else was playing with us, too. Grandpa's words echoed again and again. Ally closed her eyes, trying to remember last night's scene. In her mind, she could see Sarah, Grandpa and—

Ally dropped her pencil. She had played cards with Grandpa and Sarah!

There had been an argument. Ally had lost the game and called Sarah a cheater. Ally had stomped out of the room and refused to play anymore.

Before the argument—that was the last time Ally had seen Sarah smile!

Oops.

Ally felt terrible. She must have really hurt Sarah's feelings. But she had an idea. Ally ran to her room and pulled out markers and construction paper. When the card was finished, she slid it beneath Sarah's bedroom door. Ally returned to her own room and waited.

A few minutes later, Ally noticed her sister standing in the doorway.

Sarah's lips twitched. “Still want to play checkers?”

“Sure,” Ally said.

Ally and Sarah skipped to the living room and sat at the table. Ally looked across the table at her sister. Sarah's smile was back.

What did Ally's message say? "Dear Sarah, Sorry I said you cheated. You won. Love, Ally."