After getting an Oxygen Tank, you head into the depths of the Rainbow Dragon Cave. Now that you're able to breathe in the depths, you explore for a while, until you come across Momoyo. "Aha, a foolish human coming here, to somewhere they can't breathe. Lemme knock that oxygen tank off you real quick!" You felt scared as she said that. "Nah, just kidding!" Phew, you dodged a major bullet right there...