Staying Alert in Meditation

Once a young novice was found to be nodding off in his meditation.  His master decided to show him how to keep alert and concentrate his mind fully, without even the distractions of any thoughts.

He told the young man that they would be going for a walk.  They walked for a couple of miles, and the terrain became quite steep. Eventually they reached the very pinnacle of the climb. The master told the novice to sit down cross-legged, right on the edge, with his back to the precipice.

“Now,” he said, “I am going away for a bit – you sit here and meditate, and I will be back in an hour.  See if you can keep your mind focused till then.”

Heheh… what do you suppose the young man did?  Have a sleepy sort of meditation?