"Number sense is the foundation for all higher-level mathematics (Feikes & Schwingendorf, 2008). When students work on rich mathematics problems they develop their number sense and they also learn and can remember math facts. When students focus on memorizing times tables they often memorize facts without number sense, which means they are very limited in what they can do and are prone to making errors." (Boaler, Jo. "Fluency Without Fear: Research Evidence on the Best Ways to Learn Math Facts." youcubed at Stanford University, January 28, 2015, pg. 2.)