As much as this plot's been done, here's my take on it. xP
Seto sat at the dining table, one leg over the other, holding the paper in front of him and occasionally sipping his coffee. The most eventful moment consisted of him getting up to refill his mug. Mokuba sighed, poking his scrambled eggs with a fork.
"Mokuba, stop playing with your breakfast."
Mokuba pouted. 'Some conversation.'
Luckily, it took only a moment for him to get an idea to make his morning a little less boring. He'd stayed up late the night before when he found a great website with a list of pranks. Most of them were pretty dumb, but more than a few had invoked a laugh. Unfortunately, that made it hard to keep it from his brother, who had indeed caught on. Mokuba finally gave up, he could always read more later. He only hoped his brother wasn't too sore about it.
"Hey, Seto?" he inquired innocently, stuffing a bite of his eggs in his mouth and setting down his fork.
Seto kept his eyes glued to the paper, grunting a quiet "Hm?"
Mokuba propped his elbow up on the table, ignoring what he'd been taught about manners, and strategically placed his hand where it would block Seto's view of his mouth; it would be too hard to keep a straight face. "Where do babies come from?"
Inwardly, he beamed in praise of his performance. He half expected he wouldn't be able to get through the question. He looked at Seto's face closely. He wanted to remember it, because chances were, he'd never see it again. And indeed, Seto looked like he'd been hit over the head with a metal bat and hadn't quite realized it yet. Though metaphorically speaking, that might not have been far off. He opened his mouth and snapped it shut just as fast. Mokuba would would have told him now, but he was curious about what Seto would do. In true Kaiba fashion, he quickly put on a neutral face and cleared his throat. A less observant person might even have guessed him unfazed. "I'll be back in a few minutes," he announced. Mokuba stared at his brother's back in confusion. 'What's he doing now?'
As promised, a few minutes later, Seto came back and dropped a small stack of papers next to him. Mokuba stopped mid-bite, scanning his eyes over the top paper. "What's this?"
"You said you wanted to know where babies come from."
Mokuba's grip went limp, his fork and food falling next to the plate. Even just briefly looking over the text, he didn't think he could stomach what he had already eaten. "Uhm, Seto... I was kidding."
Seto, now once again seated, lowered the arm holding the newspaper and looked at him questioningly. "What do you mean?"
"I'm twelve," Mokuba explained.
"So?"
"When did you know?"
"Oh."