The Potty Problem is an episode of season 2.
The episode begins with Peg and Cat smelling a bad smell. Peg asks if the smell is what she thinks it is and Cat replies, "Sure smells like what you think it is, if you think it is what I think you think it is." Peg doesn't want to say what she thinks the smell is, but they both note that they can definitely smell it.
After the theme song, Peg, Cat, Connie and Big Mouth are visiting Viv's house. The Neighbor Ladies are appreciating Big Mouth and putting many yellow bows in his fur. The group then sing a happy song, until Big Mouth "goes potty" on the rug. The group complain about the smell and wonder who did it. Peg decides to find out by asking everybody in turn whether or not they did it. The Neighbor Ladies and Cat all say they didn't and Peg knows that she didn't, so she asks Big Mouth if he did it. He nods. Cat tells him to go to the bathroom instead.
When they clean up, they think the problem is solved, but Big Mouth goes potty on the rug again and Viv decides that she will not allow him to visit again if he keeps doing it. Peg points out that on Big Mouth's planet, everyone goes potty wherever they want, but on Earth, people have to use toilets. They then declare the situation a very big problem as Viv doesn't want Big Mouth to go potty on her rug but does want to have him visit. Big Mouth and the Neighbor Ladies hide under the couch, but Peg says that it is a big problem but it can be solved with potty training. Big Mouth doesn't know what potty training is, so Peg explains that everyone on Earth is born not knowing where to go potty and potty training is learning where to go potty. The Neighbor Ladies say that Peg herself was "stinky" before she was potty trained. Peg is a bit offended, but notes that she is not stinky anymore and that Big Mouth can be potty trained too. She then starts to list the steps on how to go to the bathroom, the first being "listen to your body". Connie doesn't know what that means, so Peg explains that it means interpreting physical sensations and Viv and Cat use pain and hunger respectively as examples. Peg asks Big Mouth what his body feels like and he replies, "Uh-oh", which Peg interprets as needing to go to the bathroom.
In the bathroom, Big Mouth sits on the toilet and the others stand around him. Peg goes over the other steps and states that Step Two is waiting. Big Mouth is impatient, so Peg suggests they sing a particular song to keep him entertained while he waits, but they have trouble remembering how many "boomy"s and how many "boom"s are in the song, so they write it down on a piece of toilet paper. After singing, Big Mouth finishes up on the toilet and washes his hands. Then, the Neighbor Ladies pick him up like a three-person version of crowd surfing and sing.
After that, they all get in the car to go somewhere but Big Mouth has to go potty again. The Neighbor Ladies have trouble remembering which order to press the buttons in to open the door and turn the wipers on. Eventually, they do it in the right order, and they go to the bathroom.
Big Mouth sits on the toilet and they try to sing the song again but have forgotten the lyrics again and they accidentally flushed the piece of toilet paper down the toilet. Peg and the Neighbor Ladies freak out but Cat and Big Mouth don't and suggest that Peg and the Neighbor Ladies count backwards to clear their minds. Upon the "three, two" part, Peg remembers the lyrics (three "boomy"s and two "boom"s) and they sing the song while dancing.
They dance out of the bathroom and then realize that Big Mouth can now go to the bathroom independently, which means he is now officially potty trained and they all sing a song about how as he's potty trained, they can go anywhere with him.