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Secondly , you may want to remove all minerals if you are marketing a product to children . If you ever get a chance to examine a developing hen 's egg , try ' candling ' it . In the United States , however , higher education is controlled mostly at the institutional level through internal efforts to monitor quality and by accreditation – a mostly non-governmental process for external accountability . Secondly , when you blow air out under pressure , its temperature falls as it expands . Bird bones are actually hollow ( air filled ) in the living animal , presumably for lightness , but air also circulates within their skeleton and may assist with respiration . They gradually abandoned the hunter-gatherer way of life , began to live in permanent settlements , and began to practice agriculture .