Water

"Let's try it with water" The girl said.

The guy then fill the storage with water and start the machine. The water start to flow into the girl's breast. When it made contact with the inside of her boob, the girl cooed. The breast can then be seen growing bigger than her twin. Once the storage has been emptied, the girl pull off the hose from her chest and inspect her water-filled boob. By shaking it, she could hear some audible sloshing sounds.

"I feel like I have a water balloon on my chest" The girl said

"Yeah, that funny" The guy said

The girl currently have one breast filled with water and bigger than the other. Making her chest lopsided.