It Has Your Name on It.
Disclaimer: Don't own Yugioh. I have a five-year-old brother he did this (Mokuba's part) and it just reminded me so much of Seto and Mokuba. No yaoi. I will say it once and a thousand times over. They are brothers.
"Seto, I'm thirsty."
"There's a cup of water over there with your name on it." An eight year-old Seto looked up from his book and pointed to a glass of water he had gotten for Mokuba before his brother had fallen asleep.
A few minutes later Mokuba re-entered the room, "Seto, I don't see it." He examined the cup.
"See what?"
"My name."
Seto smiled, and put down his book. He fumbled through the desk drawer until he found a marker.
"Here…."
He wrote 'Mokuba' in big letters across the side of the cup, "…now it does."