Rocks have a chance to be replaced with poop when generated.
The formula to calculate the chance is: , capped at 10% at 7.5 Luck.
Destroying Poops spawn 1-4 Dip Familiars, which loosely follow Isaac around, block enemy shots, and deal contact damage to enemies.
Dips deal contact damage to enemies equal to the damage Isaac had at the time it spawned, and can deal up to 2 ticks of contact damage and/or block 2 shots before dying.
Destroying different types of poop will spawn special Dips:
Brownie Dips spawned by Brownie and Dangle have the same behavior.
Red Poops create Red Dips that deal twice as much damage and autonomously charge at enemies, but automatically die after 15 seconds.
Golden Poops create Gold Dips that have a chance to inflict the Midas' Touch effect on enemies they hit. They can also take 5 hits before being destroyed.
White Poops create White Dips that recreate the effect of Hallowed Ground and can take 3 hits before being destroyed.
Poops unique to Tainted ??? and IBS also spawn Dips with special effects:
Stone Poops create Rock Dips that deal an additional 3.5 damage per hit and can take 6 hits before dying instead of 2.
Stinky Poops create Stinky Dips that release toxic gas as they move and a larger cloud of toxic gas when they die.
The toxic gas does 0.5x Isaac's damage per tick (~1.2x Isaac's damage per second) and poisons enemies; clouds left behind while alive last 1.5 seconds and the cloud created on death lasts 8 seconds.
Fire Poops create Flaming Dips that burn enemies they hit and leave behind a fire when they die.
The burn deals 3.5 damage up to 6 times, and the fire deals 8 damage at full durability, 6 after taking 1 hit, and 4 otherwise, for a maximum of 22 damage.
This item belongs to the Oh Crap set. Collecting three items from this set will transform Isaac into a walking pile of poop.
Notes
The familiars are destroyed by Bombs, making them very easy to destroy when trying to blow up walls or rocks.
Familiars remain if this item is lost (re-rolled, etc.).
Enemies that charge towards Isaac, such as Gurdy Jr., can deflect off of friendly Dips in unpredictable directions.
Synergies
BFFS!: Doubles damage dealt and halves damage taken by Dips, as well as increasing their size.
Butt Penny: Grants a chance to spawn a Dip each time a coin is picked up.
E. Coli/Flush!: Poops generated by these items spawn double the normal amount of Dips.
Mom's Bracelet: Friendly Dips can be picked up and thrown.
Mysterious Candy: Allows a steady supply of Dips, with time. Useful for later chapters.
The Poop: Allows generation of extra Dips after every room.
Sacrificial Altar: All Dips are sacrificed and a coin is spawned for each Dip sacrificed. The Dips do not count towards the 2 familiar limit of Sacrificial Altar.
Each use of Glowing Hourglass that does not move Isaac to a different room from the one he is currently in (such as by teleporting to the previous room then using it again) will duplicate Dip Familiars.
Dips and 
Drips into Dip Familiars.
Poops spawn 1-4 Dip Familiars, which loosely follow Isaac around, block enemy shots, and deal contact damage to enemies.
Corny Poops create Corny Dips that spawn 1 Blue Fly on death.
Red Poops create Red Dips that deal twice as much damage and autonomously charge at enemies, but automatically die after 15 seconds.
Golden Poops create Gold Dips that have a chance to inflict the 
Midas' Touch effect on enemies they hit. They can also take 5 hits before being destroyed.
Rainbow Poops create Rainbow Dips that charm enemies they hit and have a chance to drop a 
Half Red Heart on death.
Black Poops create Black Dips that leave a trail of slowing creep.
White Poops create White Dips that recreate the effect of 
Hallowed Ground and can take 3 hits before being destroyed.
Tainted ??? and 
IBS also spawn Dips with special effects:
Oh Crap set. Collecting three items from this set will transform Isaac into a walking pile of poop.
Bombs, making them very easy to destroy when trying to blow up walls or rocks.
Gurdy Jr., can deflect off of friendly Dips in unpredictable directions.
BFFS!: Doubles damage dealt and halves damage taken by Dips, as well as increasing their size.
Butt Penny: Grants a chance to spawn a Dip each time a coin is picked up.
Compost: Compost does not multiply Dips.
E. Coli/
Flush!: Poops generated by these items spawn double the normal amount of Dips.
Mom's Bracelet: Friendly Dips can be picked up and thrown.
Mysterious Candy: Allows a steady supply of Dips, with time. Useful for later chapters.
The Poop: Allows generation of extra Dips after every room.
Sacrificial Altar: All Dips are sacrificed and a coin is spawned for each Dip sacrificed. The Dips do not count towards the 2 familiar limit of Sacrificial Altar.
Bug!
Glowing Hourglass that does not move Isaac to a different room from the one he is currently in (such as by teleporting to the previous room then using it again) will duplicate Dip Familiars.
Poop 
The Binding of Isaac: Rebirth
