Luckily I had everything we needed including a brand new balance I had been waiting for just the right moment to pull out.
We first weighed two empty balloons to ensure they were all the same.
Next we blew a balloon most of the way. But we could not put it on the balance. It took lots of investigating and problem solving but we finally devised a way to weigh it without messing up the results.
Her hypothesis was right ofcourse but proving it was a fun learning experience.