Good Question!
Jill's comment: Most of the air we swallow, especially the oxygen component, is absorbed by the body before the gas gets into the intestines, so by the time the air reaches the large intestine, we are left mostly with nitrogen. Fart gas also contains carbon dioxide, produced by chemical reactions between stomach acid and intestinal fluids. However, a nervous person who swallows a lot of air may have some oxygen in his farts, because his body didn't have time to absorb the oxygen. However most farts contain no methane, contrary to popular belief. The actual components of the gas: nitrogen, carbon dioxide, and oxygen cannot hurt you and are nothing to fear. So, while smelling a fart may be gross it is not, in fact, dangerous to your health.
Magnets play an integral part in the electromagnetic circuitry that affects the electrical energy through the coil of wire in the speaker to reproduce the sound waves. Basic physics tells us that the electromagnetic force interacts with electron forces in a way that conducts electricity; this is due to the manner in which atoms in a magnet and atoms in an electrical conductor, like a metal coil, react to one another. When placed closely together the magnet sets electrons moving between atoms on the wire coil, thus conducting electricity and allowing the electronic to work.
As for part two of your question, we can all be happy to know that studies have been done to show that, not only do magnets cause no harm to the brain, they may even make the brain stronger! According to an online article from May of last year, "in mice at least, stimulating the brain with a magnetic coil appears to promote the growth of new neurons in areas associated with learning and memory. If the effect is confirmed in humans, it might open up new ways of treating age-related memory decline and diseases like Alzheimer's."
So use your magnetic headphones without worry!
For eyesight: In short, yes there are things you can do but they won't work 100%. According to the NY Times, "reading in poor light, sitting too close to the TV, wearing the wrong prescription glasses and crossing your eyes — things your mother warned you would ruin your vision — are actually generally harmless." However, patients should visit an ophthalmologist regularly, not just to get a new pair of glasses, in order to keep up with screenings for eye diseases like glaucoma which can harm vision when left untreated. Diet and exercise are also important; an adequate amount of vitamins C and E as well as omega-3 fatty acids may reduce the risk of cataracts and prevent worsening of macular degeneration, and aerobic exercise can lower pressure in the eye. Also, refraining from smoking and staying protected in the sun have also proven to have a positive effect on eyesight in studies.
Preventing crooked teeth however is nearly impossible as the cause is usually genetic conditions such as dental structures that do not permit perfect occlusion. For example, someone who inherits a small jaw from his or her mother and large teeth from his or her father, is likely to have crooked teeth and need braces or a similar treatment in order to deal with the issue. No self-care can correct improperly positioned teeth. The only thing that can be done to help the situation is, if fitted with braces, making sure to take extra care with brushing and flossing to help assure that tooth decay does not develop adjacent to the appliance.