Answer:
An image taken from real life
You fill it and it empties, A metaphor for plenty, Depending on what half you see, It's either full or empty
Men in skirts blow into these.
What is filled with air and sometimes a gas, tied to a string and often seen on your birthday?
The more I appear The less you see. So riddle me this: What could I be?
All about, but cannot be seen, Can be captured, cannot be held, No throat, but can be heard. Who am I?