Some writers have suggested that they may have evolved as a visual signal of sexual maturity and fertility.Many people regard bare female breasts to be aesthetically pleasing or erotic, and they can elicit heightened sexual desires in men in many cultures. In the ancient Indian work the Kama Sutra, light scratching of the breasts with nails and biting with teeth are considered erotic. Some people show a sexual interest in female breasts distinct from that of the person, which may be regarded as a breast fetish. A number of Western fashions include clothing which accentuate the breasts, such as the use of push-up bras and decollete (plunging neckline) gowns and blouses which show cleavage. While U.S. culture prefers breasts that are youthful and upright, some cultures venerate women with drooping breasts, indicating mothering and the wisdom of experience.Research conducted at the Victoria University of Wellington showed that breasts are often the first thing men look at, and for a longer time than other body parts. The writers of the study had initially speculated that the reason for this is due to endocrinology with larger breasts indicating higher levels of estrogen and a sign of greater fertility, but the researchers said that "Men may be looking more often at the breasts because they are simply aesthetically pleasing, regardless of the size. "Some women report achieving an orgasm from nipple stimulation, but this is rare. Research suggests that the orgasms are genital orgasms, and may also be directly linked to "the genital area of the brain".
In larger casting, they can be up to a millimetre (0.040 in) in diameter.To prevent gas porosity the material may be melted in a vacuum, in an environment of low-solubility gases, such as argon or carbon dioxide, or under a flux that prevents contact with the air. To minimize gas solubility the superheat temperatures can be kept low. Turbulence from pouring the liquid metal into the mould can introduce gases, so the moulds are often streamlined to minimize such turbulence. Other methods include vacuum degassing, gas flushing, or precipitation. Precipitation involves reacting the gas with another element to form a compound that will form a dross that floats to the top. For instance, oxygen can be removed from copper by adding phosphorus; aluminium or silicon can be added to steel to remove oxygen. A third source consists of reactions of the molten metal with grease or other residues in the mould. Hydrogen is produced by the reaction of the metal with humidity or residual moisture in the mould. Drying the mould can eliminate this source of hydrogen formation.Gas porosity can sometimes be difficult to distinguish from micro shrinkage because microshrinkage cavities can contain gases as well. In general, microporosities will form if the casting is not properly risered or if a material with a wide solidification range is cast. If neither of these are the case then most likely the porosity is due to gas formation. Tiny gas bubbles are called porosities, but larger gas bubbles are called blowholes or blisters.