What's That Smell?

We're in a car on this occasion, with 13 Year Old Sister (and friend) and 10 Year Old Brother.

I open the window. There's one of those odours one sometimes encounters in rural areas.

"Phew, who made that smell?" I joke hilariously.

"It's that fly," 5 Year Old assures us.

"What fly?" I ask.

"A fly flew up my nose yesterday," she tells us.

"Oh. But why is that making a smell?"

"Well, it's still up my nose, and it's probably farting. And I can REALLY smell it."

We laugh. She's offended. But what can you do?