When I was about 7 years old, my teacher put up a big posterboard and said when we think of questions, to write them on the board. After a week or two of this, when we each had at least one question up, our assignment was to do a project to answer the question.
My question was, "How does the body work?" The project turned into studying anatomy, the circulatory system, the nervous system, etc. But what I really meant was, "What is the essence of life?" What is that thing that makes me think, feel, want, and experience, and a table not?
The simple answer, I suppose, would be electricity. If you study the nervous system even a little bit, you understand that thinking and all that is the result of electricity being passed from one cell to another. The rift between thought and electric charges is still a great one, and I haven't studied neuroscience any more than that, mostly because I think the problem is too difficult to be solved in my liftetime. Progress might be made, and I applaud anyone who attacks the problem, but it's not my problem to solve.
Instead, I like to think about how the complexity came about. Not long after that posterboard project, my school began a years-long journey in history. By the time I left for middle school, we had traveled from the beginning of the universe, through evolution, early civilizations, the middle ages, and the renaissance, and just about into colonization of the new world. I suppose I was to young to understand evolution--I thought an individual fish actually grew lungs and legs, and became a lizard--but I got the point: over long periods of time, simple tends toward complexity. Evolution became my favorite curiosity, and when I learned about genetics years later, it was the most beautifully logical answer to the problem of evolution.
It's easy to understand how change occurs in DNA. Since Watson and Crick puzzled out the structure of DNA--two complementary strands wound in a double helix--it was obvious how the molecule could be copied*. Unzip the strands and each serves as a template for the synthesis of the other. Make a mistake, and change occurs.
Since DNA encodes genes, and genes encode proteins, and proteins do the work of the cell, a change in DNA might result in a change in the ability of a protein to do its job. Most of the time, mutations have no significant effect, in that the protein still works just as well. Sometimes, a mutation might stop it from working. Rarely, a mutation will make the protein work better. Or it will allow the protein to take on another task. Maybe the gene will be duplicated, and one copy will accumulate mutations that allow it to serve a new purpose in the cell.
On a molecular level, evolution is neatly obvious. It gets a little muddy if you start thinking on a larger scale. How could the complexity of [anything] arise when each part, individually, serves no useful purpose? How could one ancestor give rise to both chimps and people? And how did life begin in the first place?
Those are questions for another day.
*Or not so obvious! Apparently, the idea that a nucleic acid could serve as a template for its own replication was somewhat preposterous. It was not until the identification of DNA polymerase, the molecule that reads one strand of DNA and synthesizes a second complementary strand, that the copying mechanism was accepted.