And since it's potty-training we're discussing, I'm guessing it's also number 2!

Ah, nothing like a little bathroom humor!
(Very little...)
You'll fit right in at my house!
DS3 has been trying out various ways to *slide* bathroom humor into mealtime conversations. "Toilet Talk" isn't acceptable in our house during mealtimes. So, he's been trying various tacticts like telling a story about a train that "toot-toots" all the way home, etc. That explains why he and his sister spent an hour giggling over the thesaurus the other day. They are also working to combine words into a sort of code so they can weave words into the conversation that might otherwise not be allowed. "Furt" is one example. An example of the word used in a sentence would be, "At the park the other day, some kid did a furt". In fact my kids used that sentence quite a bit at the dinner table the other night, giggling often, until finally DH and I asked them what it meant. We waited until after dinner for the answer per their request.
Turns out that "furt" is short hand for "fruit fart". As DD explained it it's when a person "passes gas" and leaves "a mess" ..."like when you squeeze a grape". She didn't use the word fart in her definition, because "Mom, it's dumb to define something by using it's own name. I don't know why people do that. It's not helpful at all!"