At some place somewhere there is a small pump shaped like a rubber ball. Normally these types of pumps would have a small tube or hose attached but this one didn't. This gives the ball the impression of maybe being a stress ball, a dog toy, or perhaps just a broken cheap pump. Of course since we're here we all know it's none of those things, but the inflation ball. The inflation ball is a simple magic item, you squeeze the ball, the air in the ball gets let out, and something inflates. What exactly this something is depends a bit on the user though. If the ball gets randomly squeezed or the user isn't thinking it'll just inflate something in the immediate vicinity randomly. Thankfully for it's victims the inflation slowly goes away like how a birthday balloon will slowly loose it's helium.
No one is entirely sure how it works, but many cite finding the inflation ball in some attic or old tool box. Others say that their dog's old ball ended up becoming the inflation ball or that they bought it at a pawn shop, but it seems that there is only one active at a time. More than that destroying or locking it away somehow will just activate another inflation ball. Either nearby or far away, and the exact details of each ball seem to shift.