Comment by nate_meurer

I'm just as fascinated by the valve at the base of the stomach, called the pyloric sphincteric cylinder. It uses an ingenious mechanism to selectively pass liquid and very small particles to the small intestine, while keeping larger chunks in the stomach for further digestion. It functions like a screen or filter, but with a simple, robust, and reliable structure.

Here's a fantastic (and free) book about just this structure:

http://med.plig.org/