Well, balloons are full of air - i.e. "nothing" - so in many ways this is the perfect symbol of what we know as the ZERO ....
Zeroes are not worth much when you need something and have to find money for it on your empty bank account, but GOSH, how interesting as an idea.
"Zero's origins most
likely date back to the “fertile crescent” of ancient Mesopotamia.
Sumerian scribes used spaces to denote absences in number columns as
early as 4,000 years ago, but the first recorded use of a zero-like
symbol dates to sometime around the third century B.C. in ancient
Babylon." (History)
According to tradition it appeared in Europe as late as approximately 1200, also introducing the "comma" of numbers which has its own mystique.
When somebody asks you "is this (empty!!!) number even or odd one finds out just how strange it is. How is it even possible that it entered the series of numbers: 1 - 2 - 3 - and so on or 0 - 1 - 2 - 3, etc. when it in itself is empty? It may be tempting to denounce it as some kind of "non-number", but that would ruin the picture of how to do mathematics as well as a lot of other things in the modern world ....
https://www.history.com/news/who-invented-the-zero
https://www.cbc.ca/radio/ideas/zero-number-series-ideas-cbc-1.6977700
https://www.popsci.com/science/crows-can-count/?utm_term=pscene052324
Britannica
Wikipedia