On that particular Saturday, sisters Bong Bong and Trang Trang, both daughters of Mr. Nga Nga, visited their grandfather's house and decided to stay the night. Grandfather inquired:

"Who will sleep next to me tonight?"

All three replied simultaneously:

"Bi Bi."

Pressing further, Grandfather asked Bi Bi:

"Given that you are with me every day, who will sleep next to me tonight?"

Bi Bi muttered something indistinctly and then fell silent. Grandfather pointed out:

"There are only two spots beside me, yet there are three grandchildren. Thus, one child must yield for the others. Will Bi Bi yield?"

Reluctantly, Bi Bi agreed with a muffled response. After arranging the sleeping arrangements, Grandpa retired to his room to work, as was his nightly habit. Meanwhile, carefree Trang Trang enjoyed a peaceful night's rest, while energetic Bong Bong struggled to fall asleep. Suddenly, Bong Bong heard Bi Bi sobbing loudly. Concerned, Bong Bong asked:

"What's wrong, Bi Bi?"

"I have a stomachache," came Bi Bi's reply.

Bong Bong woke Trang Trang, and the three sisters headed to Grandpa's room. Bong Bong informed Grandpa:

"Sir, Bi Bi is in pain."

Grandpa inquired:

"How does your stomach feel, Bi Bi?"

"I don't know."

"Then we should take you to the hospital."

"No."

After observing Bi Bi for a moment, Grandpa decided:

"Both older children should return to bed; I'll take Bi Bi to the hospital. Bi Bi, get ready, put on some warm clothes, and I'll take you."

Panic-stricken, Bi Bi exclaimed:

"No, I don't want to go to the hospital."

Grandpa gathered the three grandchildren on a cozy cushion and engaged them in conversation for a while before asking Bi Bi:

"Feeling better now?"

"I'm fine," replied Bi Bi.

Grandpa, with a twinkle in his eye, remarked:

"You're quite the actress. Now, Bi Bi, tell us what really happened last night."

In unison, the sisters urged:

"Yes, Bi Bi, tell us."

Confused, Bi Bi hesitated to speak. Mr. Nga offered an insight:

"When Grandpa suggested letting her sisters sleep next to him, Bi Bi felt a pang of jealousy, thinking, 'Why can't I be the one?' Is that correct?"

Bi Bi quickly affirmed:

"Yes."

"At that moment, Bi Bi suddenly became...?"

"Selfish," Trang Trang answered promptly.

"So Bi Bi felt a bit envious of her sisters, right? That's why Bi Bi...?"

"Was crying," Bi Bi admitted.

Encouraged, Bi Bi added:

"I cried and then stopped, and then cried again..."

Bong Bong added:

"After that, Bi Bi approached Grandpa."

Trang Trang added:

"We asked her why she went to Grandpa, and Bi Bi continued crying."

"Who mentioned the stomachache?" Grandpa inquired.

"I did," Bi Bi confessed.

With a serious tone, Grandpa instructed:

"Now, Bi Bi, stand up, fold your arms, and tell the truth."

Realizing Grandpa's understanding, Bi Bi expressed remorse:

"I wanted to be beside you, but I lied about having a stomachache. I won't lie anymore."

Turning to her sisters, Grandpa asked:

"Is that acceptable?"

"It's fine," they replied.

"Will you forgive Bi Bi?"

"We forgive her!" they chimed.

Seating Bi Bi, Grandpa advised:

"From now on, just tell me what you want. If you want to be beside me, ask, and I'll let each of you take turns."

The next morning, Bi Bi questioned:

"Grandpa, how did you know I was lying?"

"I can tell by looking at you."

Impressed, all three sisters exclaimed:

"You're like a wizard! You know everything."

However, Bi Bi persisted in understanding how Grandpa discerned her lies. She asked repeatedly:

"How did you know I lied?"

Gathering them close, Grandpa whispered:

"Lying isn't right. It's hard, and people can tell. Especially me, who knows you well. It's best to tell the truth."

Addressing Bi Bi directly, he asked:

"Who do I belong to?"

"You belong to us."

"That's right. I belong to all of you, so I love you all. But since your sisters live far away and visit less often, they deserve priority. Do you agree, Bi Bi?"

"I agree."

The four grandchildren shared a joyful laugh. Just then, Grandma called:

"Come down for breakfast!"

The aroma of freshly baked donuts filled the air, accompanied by the warmth emanating from the kitchen, dispelling the winter chill.