Especially when they do things like demand a clean fresh diaper so that they can poop in it immediately. Or insist that the diaper must be changed right now so that they may pee all over the place during the diaper change.
Or (my favorite) when they decide to pee, poop and vomit all at the same time while the diaper is off during changing. And Mommy's hands are busy dealing with one of these so she can't attend to the other two right away. And the baby, who is now covered with milk curds, takes advantage of Mommy's busy hands and wiggles around, getting poop all over his feet and his sleeper. And then he smiles and giggles at Mommy, all proud of his tremendous skill.
Babies are a lot of work. But no matter what their antics, and no matter how icky their various messes may be, that smile and giggle just mean the world. J smiles at me, and my heart melts and all of my frustration just disappears. He is such a sweet little boy, and I love him so much!