What do you mean by spare cylinder?

Our carbonating cylinders are the heart of your Sparkling Water Maker. It perfectly fits your Sparkling Water Maker and contains colorless, odorless, incombustible gas, used to help create the bubbles.

A spare cylinder is there so you never run out of gas. You pay for the cylinder and the gas, and there's no need to return an empty cylinder. SodaStream recommends having at least 3 cylinders on hand, so you’ll always have bubbles . Get your back-up CO2 cylinder here.