Bubbles 101: Carbonating cylinders create the bubbles. After some time, the carbonating cylinder runs out of CO2. We offer an exchange in which you get a new carbonating cylinder in exchange for your empty one. Once you order your exchange at SodaStream.com, our Gas Refill program has 3 easy steps to get you back to bubbling. 1. We'll send you full cylinders and include a prepaid USPS label with your order. Make sure to remove the label from the box and record the tracking number on the label for your records. 2. Place your empty cylinders in the same box in which you received your new cylinders and affix the label to the box. 3. Drop the package in the mailbox or at a local post office. You can also schedule a free USPS pick-up (if applicable to your location). The shipping is on us. Don’t wait too long to send back your used cylinders.** *If you have 3 or more cylinders to exchange, you will receive a UPS label via email and the return package with your empty cylinders should be dropped off at your nearest UPS location. **Remember to send in your used cylinders after placing a cylinder exchange order. SodaStream must receive your used cylinder(s) no later than 30 days after placement of your exchange order, or an additional $15 charge (plus tax) for each unreturned cylinder will be applied. Make sure to return an empty cylinder for each refill cylinder you have ordered to avoid any additional charges.