When it reaches zero a new row of bubbles is added and health is restored.
Every time you miss a shot your health will go down by one. In other games they advance after you fail to make a match for a number of shots.
In some games the bubbles will slowly move down the screen for instance in Descending Balls (that one is an older flash game so it will only work on PC). Keep popping bubbles and last as long as you can before you get game over. Here are some of the key variations of bubble shooter game that you will find on : Endless Bubbles
Even if you have a favorite game it is still nice to play a new variations to enjoy a fresh challenge.
