Bubble

Bubbles were first introduced in Angry Birds Space, and it is currently unknown if it would appear in other series of Angry Birds. When found, a pig is usally inside the bubble, used for the main purpose so that the pigs can breathe in space, much like an astronout's helmet. Lights can be usally found on the bubbles, like in level 3-10 in Angry Birds Space

In the Game
As mentioned above, the Pigs use the bubbles mainly for breathing in space, but are also used to float around. If a bubble is destroyed and the pig is not in an atmosphere, the pig will freeze. The frozen pig will then explode after a few seconds if immobility. If, however, the bubble is popped in an atmosphere, the pig will not freeze, but will most likely die from the planet's gravity by hitting the planet too hard.