Question: when a cork is added to a glass of water

title: Water fluoridation
text: A 2000 UK systematic review ( York ) found that water fluoridation was associated with a decreased proportion of children with cavities of 15 % and with a decrease in decayed , missing , and filled primary teeth ( average decreases was 2.25 teeth ) . The review found that the evidence was of moderate quality : few studies attempted to reduce observer bias , control for confounding factors , report variance measures , or use appropriate analysis . Although no major differences between natural and artificial fluoridation were apparent , the evidence was inadequate for a conclusion about any differences . A 2007 Australian systematic review used the same inclusion criteria as York 's , plus one additional study . This did not affect the York conclusions . A 2011 European Commission systematic review based its efficacy on York 's review conclusion . A 2015 Cochrane systematic review estimated a reduction in cavities when water fluoridation was used by children who had no access to other sources of fluoride to be 35 % in baby teeth and 26 % in permanent teeth . The evidence was of poor quality .