What is the sound of one hand clapping?

Where does the ocean end and the wave begin?

These are examples of koans. 

A koan, in Zen Buddhism, is a paradoxical question meant not to be solved logically so much as contemplated until it opens the mind to a deeper reality.

Barbara Brown Taylor, in her book Home By Another Way, describes the doctrine of the Trinity as a Christian koan.

How can one God inhabit three forms?
How can God be both three and one?