Telling Elizabeth wasn't going to hurt anything; they already teased one another about being weird all the time. Besides, it's not like the situation could have gotten any weirder by telling her about it.
"Um, Lizzie..." Monica began, trying to figure out what to say. "A moment ago, after you left, I mean..."
"Wait, what?" questioned Elizabeth.
Monica knew this was going to be a lot harder to explain than she initially thought. "Well, something happened... with the balloons."
"Girl, you're not making any sense. Did one pop? We've got plenty more if that's what you're concerned about."
Monica took a moment to breathe deep, ready to explain. "While you were gone, one of the balloons I was blowing up was giving me a hard time. It took a few blows before it finally started to inflate. As I went to take another breath, I lost my grip on the balloon and the air went... into me." She paused, "...and my stomach blew up like a balloon."
Elizabeth was confused. "...say what now?"
Words weren't working, and Monica could only think of one other way to explain - a demonstration. Monica grabbed another balloon and inflated it completely, but pinched the neck rather than tying it off. "OK, Lizzie. Watch this..." Making sure her lips were firmly over the balloon, she released the end of it, letting its entire volume flow straight into her. As with last time, the air inflated her stomach, her pot belly firm and round again. "See?"
Elizabeth was at a loss for words... she just stared. After a brief, awkward silence, she spoke. "How did you do that?!"
"It was an accident. It just happened... I dunno."
"Does it hurt? Can you do it again?"
"No, it doesn't hurt. It actually doesn't feel half bad," Monica stated as she unconsciously began to rub her swollen belly again. "I don't know if I can do it again. Right before you came back in, I started to blow up another balloon. My stomach deflated as I blew it up."
Elizabeth was actually more intrigued than weirded out. "OK, we should try to figure this out, so let's set up an experiment, see if we can make any sense out of this." She reached for one of the already inflated balloons on the floor, untying it but holding it closed. "Test number one..."