Kaiba lifts a brow when he sees what's inside. He would ordinarily never eat anything even close to this level of sugar bomb, but with Xander's hopeful smile, he relents. Box open, small piece cut. It is, indeed, a total sugar bomb.

"Guess the train knew if there was anything you needed, it's more sugar? That looks like a unicorn spit it up?" It's a gentle, teasing joke...or at least that's what he intends. He's trying to be calm here.