Ghost Bricks

You may be looking for Grey Blocks. Ghost Bricks are a kind of block introduced in Haunted Hogs.

Usage
Ghost Blocks are square, translucent blocks. When damaged, they split evenly into four smaller Ghost Blocks. They are used to support blocks in a structure. However, they are invulnerable and transparent to Birds, and Pigs, as well as TNT Crates. The player can use this advantage to throw Birds through areas they normally couldn't. However, this also serves as a disadvantage. With the exception of Stella, the blocks are completely invulnerable to Birds and their abilities, making stages that are mostly made of Ghost Blocks more difficult. All the birds can fly in them and never destroy them. If a structure, however, falls onto a few ghost blocks, only some will be able to enter.

Trivia

 * In The World of Angry Birds Official Guide, the Mighty Eagle said that these blocks are made of pure ectoplasm. An ectoplasm is a supernatural viscous substance that is supposed to exude from the body of a medium during a spiritualistic trance and form the material for the manifestation of spirits.

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