A Difficult Code

Lita had finished putting her pajama top on, then Akio, Mamoru, & Wataru entered the bedroom with the two sets of bunk beds in it.

"What do you make of this?" Wataru asked Lita.

"I don't know yet." Lita said to Wataru.

Mamoru saw Lita trying to reveal the writing on the envelope with a number two pencil.

"Lita?" Mamoru asked.

"What is it, Mamo?" Lita asked her younger sister.

"I don't think that's going to work." Mamoru said doubtfully to Lita.

"What do you want me to do?" Lita asked Mamoru logically.

Wataru handed something to Lita, "Here, use my special pen." he said.

Lita just then used Wataru's pen and scribbled on the envelope revealing her name in green ink surrounded by blue ink, "Aha! I found something!" she exclaimed.

"What did you find, Li?" Wataru asked.

"Simple, Wata," Lita said. "whoever wrote this letter wrote it with an invisible ink pen, just like this one."

"Invisible ink, Li?" Mamoru asked.

"That's right," Lita said to Mamoru. "now the question remains."

"What question?" Akio asked Lita.

"Which one of us children, aside from Wataru that is," Lita asked her three siblings. "would write secret messages with an invisible ink pen?"

"There is only one rational way that question can be answered." Wataru answered.