25cm Balance Mini Yoga Ball, Elbourn Small Pilates Ball Fitness Exercise Stability Pilates Ball Anti-Burst Physical Ball

I bought this so that my 2 yr old son could sit on it instead of taking my giant one. To inflate it they give you a piece of plastic straw. Like the ones you get from restaurants. You then stick the straw into the ball and blow. You have to move fast so you don't lose all the air you just put inside.

I bought it for placing between my knees for squeezing them together during my hernia exercises. I use it barely inflated for maximum motion together to the resistance I want. As such it is about 7 inches just expanded to round which works just fine for me. To inflate it much more to get it to the 25cm claimed size you may have to leave the valve cap min and push a needle into the hole between the cap and the ball while leaving the cap in place. That should be easy to do as the ball is very flexible, which I like.