For example (only an example, of course this would not happen in my house):
You are bathing your young son. He's splish-splashing in the bathtub, and you're trying to wash him. He's sitting on his legs, so you can't get to them. You ask him to move his legs, so you can just wash them already. He sticks his butt up and let's one rip.
Yes, "one", a fart, gas, cuts the cheese. It happens underwater and bursts into a big bubble at the surface. And oh it smells. Ugh! You say something to the effect of "Eww, that was gross." Then the sweet little boy of yours says "I was trying to fart on your hand."
Where do they get this stuff?
Why is it so funny to them?
Then when you may or may not have told the story to your husband, he just smiles and laughs.