Heading into the summer holidays, many people like to have a big, consuming hobby to while away the time. This issue’s features lend themselves to several such do-it-yourself endeavors, if you are suitably ambitious.

Build a solar system (difficulty: 9). The great thing about this project is that although it requires considerable setup and the outcome is uncertain, it involves essentially no intervention later—just sit back and watch what happens. Take a gigantic cloud of hydrogen laced with traces of heavier elements, let a star or two coalesce in the center and stir the remainder just enough for a protoplanetary disk to form. “The Genesis of Planets,” by Douglas N. C. Lin, has all the details.

Astronomers used to debate whether the worlds of our solar system arose from a massive sheet of gas ripped out of our young sun during a near encounter with a passing star; that extended filament then supposedly clumped into planets. Later the favored explanation came to be that the planets were more peripheral products of the same spinning cloud that gave birth to the sun. Both those explanations involved comparatively orderly processes, with the planets taking shape in roughly the same orbits we see today.

The most recent evidence, however, reveals the heavy influence of creative chaos. When planetesimals collided, sometimes they cohered into bigger ones and sometimes they split anew; newly formed planets that were not lucky enough to find stable orbits cycled down into the sun or were flung deep into interstellar space. These discoveries might help explain some of the strange globes circling other stars that astronomers have located over the past decade, such as some super-size “hot Jupiters” that are unexpectedly close to their suns.

Invent a complex organism (difficulty: 4). Take an existing animal, then experiment with altering the genetic program that controls its embryonic development to achieve the new body plan you desire.

Hobbyists daunted by the technical complexity of that challenge should feel encouraged that nature accomplishes it routinely. Indeed, that mechanism seems to generate much of the physical variation that defines and gives rise to new species. Genomic science has found that many of the genes that build a developing body are almost identical across a wide range of highly diverse animals. What makes a horse different from a tiger, a mouse and a walrus is the set of regulatory switches in the DNA that dictate where, when and for how long those genes are active. The modern synthesis of evolutionary biology with reproductive biology is flourishing under the snappy name of “evo-devo,” as Sean B. Carroll, Benjamin Prud’­homme and Nicolas Gompel explain in “Regulating Evolution."

Rewire your nervous system (difficulty: 0.5). This one’s easy. Light a cigarette, take a few drags and voilà! For some people, that one brief exposure is all it takes to alter the brain’s systems for controlling cravings and to set up a lifelong weakness for nicotine. Joseph R. DiFranza describes how that might be possible in "Hooked From the First Cigarette." Of course, too many people, particularly children, are already doing this to themselves all the time.