I'm flat when I'm new. I'm fat when you use me. I release my gas when something sharp touches me. What am I?

Why this works

This riddle plays with the different states of a balloon throughout its lifecycle. When a balloon is new, it’s flat and easy to store; however, once it's inflated with air or helium, it becomes "fat." The clever twist comes from the last clue—when something sharp, like a pin or a needle, pricks the balloon, it releases the gas inside, causing it to deflate. This combination of meanings makes the riddle both fun and clever!