https://ja.m.wikipedia.org/wiki/%E8%A8%80%E8%91%89%E9%81%8A%E3%81%B3

https://tree-file.com/lifehack/language/

http://crd.ndl.go.jp/reference/modules/d3ndlcrdentry/index.php?page=ref_view&id=1000113449

https://yomidr.yomiuri.co.jp/article/20120620-OYTEW61295/

https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1254836697

https://ja.m.wikipedia.org/wiki/%E5%BA%A6_(%E8%A7%92%E5%BA%A6)

http://q.hatena.ne.jp/touch/1149514054

https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q1216118597

https://ja.m.wikipedia.org/wiki/%E3%82%A8%E3%82%B9%E3%83%8B%E3%83%83%E3%82%AF%E3%82%B8%E3%83%A7%E3%83%BC%E3%82%AF