Shoot and pop as many bubbles as you can by clicking on two or more adjacent bubbles to remove them. Upgrades available after the level! Play the Campaign to buy upgrades by earning experience and completing small objectives, you will also unlock Arcade mode when you return to the main menu from any of the campaign levels and any upgrades you have acquired will still be available to use. There is a chance that Iron bubble will form on your cannon every time pressure reaches 100% but you can destroy them by using the bomb bubble or the electro bubble