Belly Jelly

Effects

Enemies that touch Isaac will bounce off him and propel in the opposite direction. If they hit an obstacle or wall while bouncing, they will take damage.
- If an enemy bounces into another enemy, the second enemy will also take damage.
When an enemy bounces off of Isaac, there's a 50% chance that Isaac won't receive contact damage.
Additionally grants a 50% chance for projectiles to bounce off of Isaac without taking damage.

The damage enemies receive when they get bounced into a wall or an obstacle is equal to 8 + 2 * floor.
- The same damage value applies when an enemy gets bounced into by another enemy.
Enemies that don't deal contact damage cannot be bounced off.
Bounced off projectiles deal a flat damage that doesn't scale with the current floor or Isaac's stats. This damage is usually 6, but can slightly vary depending on the enemy that shot the projectile.
- Lasers cannot be bounced off.
The bouncing effect also works against (Fire Place) Fire Places and spiked enemies such as (Poky) Poky, (Slide) Slide, and (Grudge) Grudge, granting a chance to avoid unintentional damage.
Isaac can also bounce off (Fly Bomb) Fly Bombs spawned by (Blaster) Blasters and even deny damage from their explosion.
(Bomb) Bombs and (Troll Bomb) Troll Bombs cannot be bounced off.

Aries: Retains the chance to bounce enemies while preventing damage simultaneously.
Book of Shadows / Unicorn Stump: Can be used as a powerful offensive item since Isaac can still bounce off enemies while invincible.
Cone Head / Host Hat / Infamy: Offers an additional chance to negate damage.
Gnawed Leaf: The bouncing effect works while Isaac is invincible.
(Panic Button) Panic Button: The active item will still be activated if Belly Jelly blocks damage.
Serpent's Kiss: Allows Isaac to bounce enemies and poison them at the same time, with a decent chance to get a lot more health back in the form of (Black Heart) Black Hearts than originally spent.