Water.

Kenshin’s eyes widened. Heiji was drowning? It wasn’t possible; the only water in the room was what was left over from dinner, and that wasn’t nearly enough to drown in even if Kenshin hadn’t been watching him like a hawk for the past several minutes. Was this what he was supposed to protect the boy from? Drowning in air?

“Open your mouth,” Kenshin said, trying to keep Heiji calm. “There’s no water, Heiji. Open your mouth and try to breathe.”