No. 2

No. 2 is a passive item added in the Binding of Isaac: Afterbirth expansion.

Effects

 * Continuously firing or charging tears for 3 seconds causes Isaac to drop a Butt Bomb. Only 1 Butt Bomb can be dropped per charging cycle, so you'll need to stop firing and then charge again to drop another Butt Bomb. Taking Damage will interrupt the cycle.
 * Butt Bombs explode for 50 damage like normal bombs, but also deal 10 damage to all enemies in the room and daze them for a short period of time.
 * Essentially grants unlimited bombs, as you can just reset the charge time over and over to bomb tinted rocks and to get into Secret and Super Secret Rooms.

Interactions

 * Bobby-Bomb - Butt bombs will home in on enemies.
 * Marked - Holding the fire key to move the crosshairs for 3 seconds drops a butt bomb.
 * Sad Bombs - Butt bombs will also shoot 10 tears off in every direction.
 * Pyromaniac - Butt bombs regenerate health.

Seeds
1A8Z 43DC (Treasure room adjacent to spawn)

Trivia

 * Number Two is a euphemism for defecation.