https://dentalsignal.com/what-is-periodontal-disease-and-how-to-prevent-it/
https://www.colgate.com/en-us/oral-health/life-stages/adult-oral-care/looking-after-your-teeth-five-new-years-resolutions-for-a-healthier-mouth-1214
http://www.webmd.com/oral-health/ss/slideshow-foods-stain-teeth
http://www.webmd.com/oral-health/guide/dental-health-dental-care-products#1
http://www.medicaldaily.com/does-tea-stain-your-teeth-6-surprising-foods-cause-yellow-teeth-320442