I found this on Zen Koans. A great resource, translated into English from the book Shaseki-shu or Collection of Stone and Sand written in the thirteenth century by Zen teacher Muju. A good explanation of what it means to be mindful.
Zen students are with their masters at least ten years before they presume to teach others. Nan-in was visited by Tenno, who, having passed his apprenticeship, had become a teacher. The day happened to be rainy, so Tenno wore wooden clogs and carried an umbrella. After greeting him Nan-in remarked: “I suppose you left your wooden clogs in the vestibule. I want to know if your umbrella is on the right or left side of the clogs.”
Tenno, confused, had no instant answer. He realized that he was unable to carry his Zen every minute. He became Nan-in’s pupil, and he studied six more years to accomplish his every-minute Zen.