|Floors of Appearance||Cellar|
|Base Health||20 (When Full)|
|Deals Contact Damage||No|
|Type of Tears||None|
The Sack is an enemy in The Binding of Isaac: Rebirth that is commonly found on The Cellar floor. It is a large, white, swelling egg sack.
- The Sack is immobile. It starts off very small, but quickly grows. Its size represents both its health and how close it is to firing. When the Sack is fully grown, it will occasionally shoot out a Spider in a random direction. The Sack constantly grows until it is either its full size or killed. When damaged, it shrinks in size. Sacks are commonly spawned by spider-themed bosses and enemies.
- The player should kill Sacks as soon as possible as they are weakest when the room is first entered. However, if multiple Sacks should happen to grow to their full size, the player should balance the sizes, not allowing any Sacks to spawn any Spiders.
- Only Champion versions of Sacks can deal contact damage.