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When I was 12, Mrs. Engels lived next door. She worked as an electrical engineer at the Oak Ridge Nuclear Energy Laboratory on the other side of town. In late spring and summer, she would spend her evenings and weekends working in her garden. When she wasn't weeding or picking vegetables or petting her dog, she might lie and read in her hammock that swung between two small maple trees. I loved to visit her yard and watch the vegetables in Mrs. Engel's garden sprout, grow, flower, and ripen. She never seemed to mind if I interrupted her with questions either. Often, the questions led to stories in which she would share with me the habits of bugs, birds, and vegetables. I guess you could say I was a curious pest, and she was very patient and never made me feel self-conscious.

One day she waved her hand to get my attention and asked me to run inside my house and bring back a metric ruler. I ran into our house, rooted around in my dad's desk drawer, found one, and returned to the garden. When I arrived, all out of breath and excited about the promise of a new adventure, Mrs. Engels closed her book and sat up in her hammock. She placed the ruler On the book, paused, and began to concentrate. Slowly, carefully she placed her thumb down on the ruler. Then she placed her first finger on the ruler exactly one centimeter from her thumb. (I knew it was one centimeter. Mrs. Engels always talked out loud about what she was doing, especially when I was around. It was like being able to see her mind work.) Then, she slowly held up her hand, her fingers apart just so, and asked me, "What do you see between these two fingers, Jenny?"

"Nothing at all, Mrs. Engels," I said, peering closely at the space between her fingers.

"Try harder," said Mrs. Engels.

"Still nothing…except maybe your garden?" I guessed.

"Good. Now imagine a small cube between my fingers, exactly one centimeter on each side, with the thinnest of thin wire edges keeping it perfectly square. Can you imagine that?"

"Yes, I guess so," and I'm sure my nose was really squinched up, because it always got that way when I look at something very hard—like in my mirror at home.

"Good," she said. And carefully she placed the cube on her book and pulled a pencil out from behind her ear. She wrote a very long number on the piece of paper, and then she said, "In that cube there are 26,880,000,000,000,000,000 (She said each zero out loud.) gas molecules all bumping into each other, moving around inside and in and out of that invisible cube." She moved the cube closer to my face and asked again, "Can you see any of those molecules?"

I had to be wrinkling my nose up pretty good by this time, but I still couldn't see anything but her garden. I had to be able to see something. I mean all those molecules…but I still had to say, "Nope."

"Well, they're there. And every time we breathe, we breathe in a couple of thousand cubes full of these molecules. Do you know what molecules are?"

"Yes," I kind of lied, having heard about molecules in school but never having seen one.

I knew Mrs. Engels knew I didn’t have the faintest idea what a molecule was because she chuckled. But she nodded, wisely, and went right on and asked, "Can you guess how many different types of gas molecules there are in our cube?"

At this I was at a total loss and admitted it.

Mrs. Engels never seemed to mind if I didn't know something. She never made me feel stupid. She'd just smile, nod, and continue, "Ten different types of molecules, maybe more. It's hard to tell. But it's a clear day, so I'd guess ten. Do you know which three types are the most important?"

I took a stab at this question, "Oxygen?"

"Good," she said. "And?"

Again, I didn't know. So I shrugged.

"OK. Nitrogen and carbon dioxide. The plants need carbon dioxide to make more oxygen. But why is there so much nitrogen, do you think?"

"I don't know. I don't remember hearing about nitrogen in science class," I said.

"Exactly," she said, "we don't need nitrogen, but again the plants use nitrogen, which they take in through their roots from the ground. There's another reason for nitrogen as well. And whoever or whatever created the Earth must have known about it all along. Without nitrogen, if someone lit a match, pure oxygen would explode and burn the Earth and everything on it to a cinder. The miracle of air lies in the very special mixture of almost 78 percent nitrogen and 21 percent oxygen—just the right mix to keep the air from igniting. There are also tiny amounts of four or five other gases, like argon and such, and some dust and water vapor. But it’s such a nice dry, still day, I don’t think there’s much water vapor. Now, if I give you this cube, will you do me a favor?"

"OK," I said, not knowing what was coming next, but trusting her just the same.

"Take care of the gases in that cube as long as you can. Try to keep the mixture clean! There are almost as many molecules in your one cube of air as there are stars in the universe. Even with all the cubes of air that surround our Earth, if we don't watch out, the precious gases in your cube can become all mixed up and polluted with all kinds of dirt particles and toxins."

With that she reached out and carefully placed the cube in my upturned hand. I could not see it, nor feel it, but there was something about Mrs.Engels that made you believe in what she said.

That's how I learned about air. How about you?

1 m3 = 1,000,000 cm3
1 m3 = 2.688 x 1025 air molecules	25,880,000,000,000,000,000,000,000
1 cm3 = 2.688 x 1019 air molecules	26,880,000,000,000,000,000
1 mm3 = 2.688 x 1013 air molecules	26,880,000,000,000
Compare the above numbers to the estimated number of stars in the universe.
10 x 1022	10,000,000,000,000,000,000,000