Poop.

The babies tend to poop at the same time, which is awfully convenient.

Here are Miss Boo's observations on the poop:

Boo: Chinese baby poop is so much better than American baby poop.
Me: Why is that, Boo?
Boo: Chinese baby poop isn't sticky and it's easy to clean up. American baby poop is sticky. It sticks to your skin and burns and causes rashes. Everyone should poop like Chinese babies. But we're English, so our poop isn't as good.