One of my favorite koans... it can be difficult to contemplate, but it is really striking :

A monk asked Hsueh-feng "How is it when the ancient stream is cold from the source?"

Hsueh-feng said, "When you look directly into it, you don't see the bottom."

The monk asked "How about one who drinks of it?"

Hsueh-feng said, "It doesn't go in by way of the mouth."

The monk recounted this to Chao-chou. Chao-chou said, "It can't go in by way of the nostrils."

The monk then asked Chao-chou, "How is it when the ancient stream is cold from the source?"

Chao-chou said, "Painful."

The monk said, "What about one who drinks of it?"

Chao-chou said, "He dies."

Hsueh-feng heard this quoted and said, "Chao-chou is an ancient buddha; from now on, I won't answer any more questions."