On the first day of what would become 16 years of teaching outdoors, I stood at the edge of a defunct granite quarry watching third, fourth, and fifth graders scrabble up and slide crab-like down the steep sides of the pit. Students swung exuberantly out from a rock ledge near the bottom on a thick, woody vine that dangled from the canopy of a sycamore tree a hundred feet above.

As I pondered how to use the active and enthusiastic engagement before me as the thin end of the wedge in a unit of study on "ecosystem," a student approached me with a leaf that had an odd protrusion from its smooth green surface. "What is it?" would become my favorite question.

I didn't know. A number of students gathered around to see the peculiar leaf. A contingent of third graders hunted down several similarly disfigured leaves. Was the protrusion part of the leaf? Why would some leaves on a tree have protrusions and others not? Was the leaf diseased? How could we tell? We identified the beech tree leaves but could find none with protuberances in our field guides.

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Connection was a conceptual way in. We gathered facts. We learned anatomical terms for leaf parts, such as veins and margins. We learned to recognize the leaves and identified trees in the vicinity of the quarry. The green hues of the forest gradually resolved into sycamore and beech, herb layer, shrub layer, and understory.

We began to understand relationships among the plants that inhabited the quarry. Conditions on the floor of the pit were different from those at the rim. Flora that could not abide the periodic flooding, that on rainy days inevitably occurred at the quarry's bottom, did not survive in that location.

Building a framework for understanding ecosystem by gathering facts and using a concept, such as connection, as a lens through which to examine the facts, can be applied to many subjects. But such analytical, hands-on and minds-on engagement in outdoor education is vital.

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Human beings once lived outside and retreated to shelters on an as-needed basis. Today we talk about living in our homes and offices. Nonetheless, powerful and patient, the natural world lurks beneath our city streets and sidewalks. We ignore the natural foundation of the human condition at our peril. Thus, there is a practical purpose for outdoor education.

No matter how seemingly removed from nature our lives are or may become - as air-breathing, water-drinking, plant- and animal-consuming participants in global and local ecosystems, it behooves us to know the interactions and understand the elements of our world. Look what just a few degrees' change in the average temperature worldwide indicates.

When students in a science class adopt a tree, the pride they feel in sharing about the specimen they have drawn, researched and spent time with is palpable. The joy of engaging with the natural world can be visceral and primal. There is intrinsic value in knowing our world - forest and trees.

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Love of such study, like so much of what is learned in a lifetime, seems to me to come into being most effectively when ushered in by a teacher of children in the great outdoors.

In the course of our ecosystem study I came to know that what seemed to be unusually shaped leaf growths were actually insect galls. Leaves are a great medium for insect egg-laying, and the eggs come in a delightfully wide variety of shapes and colors.

I am hopeful and even optimistic that humankind will extend its knowledge and understanding of the natural world to the stars, and that people may populate our universe long into the future. But it is comforting to me to acknowledge that we are but one expression, one manifestation of the natural world.

A world that seems likely to fold in on and remake itself periodically, ever becoming the habitat for interesting and sometimes mysterious manifestations of life.

Michael Zimmerman is head of school at Friends School Haverford, a private, Quaker elementary school near Philadelphia. He wrote this for The Philadelphia Inquirer.