Jillian walked into the classroom. “Good morning, Ms. Kryzanski,” she started to say. But then she stopped. It was not Ms. Kryzanski standing by the blackboard. It was a substitute.

“Ugh,” thought Jillian. Subs were the worst. They never knew what to do with the kids who made noise. And they always seemed confused by the lesson plans.

When Jillian looked again, she realized that there was something different about this substitute. She was wearing a mask. It was like she was going to a costume party instead of teaching elementary school. Something weird was going on.

“Good morning, class” said the substitute. “I am Ms. Terry, and I’ll be your teacher today.” She spoke is a strange high-pitched voice.

“Why is she using that voice?” Jillian whispered to Henry.

“I have no idea. Why is she hiding her face?” Henry whispered back. It was a mystery.

Ms. Terry was still talking. “If you listen closely, maybe by lunchtime you’ll figure out who I really am,” she said.

“Whoa,” said Jillian. “It really is a mystery.”

Ms. Terry gave a math lesson. “Five is the best number,” she told them.

Then she gave a spelling lesson. “My name is the hardest spelling word,” she said.

The last lesson before recess was science. “A control in an experiment is something that doesn’t change, ” Ms. Terry explained. “Every good experiment needs a control.”

During recess, Jillian and her friends thought about the clues that Ms. Terry had given them.

“I’ll bet she’s Aaron’s mom,” said Henry. “She has a really high voice.”

“Nah, she’s Principal Perry,” said Josh. “She had a rash on her face last week and she probably needed to wear a mask.”

“What about what she said during math?” asked Bellana. “Five is the best number. Maybe she’s got five kids? Ms. Page the librarian has five kids.”

“That’s a good idea,” said Max. “But I was thinking about what she said during spelling. She said her name was the hardest spelling word. Page isn’t very hard to spell.”

Recess was almost over. In three minutes it would be lunchtime. Ms. Terry would be disappointed if they hadn’t solved the mystery.

“I’ve got it!” said Jillian suddenly. “It’s our real teacher, Ms. Kryzanski!”

“How do you know?” asked Henry.

“Well,” explained Jillian, “Ms. Kryzanski has five cats. Her name is the hardest spelling word. But most importantly, think about what she told us during science. A control in an experiment is . . . .”

“SOMETHING THAT DOESN’T CHANGE!” said all the kids together.

“We figured it out just in time,” said Jillian.

Ms. Terry met them at the door. She took off her mask and was Ms. Kryzanski again. “Well done, class!” she said. “You solved the Ms. Terry!”