Abstract— Children from low-income backgrounds enter school with much less mathematical knowledge than their more affluent peers. These early deficits have long-term consequences; children who start behind generally stay behind. This article describes how a theoretical analysis of the development of number sense gave rise to an intervention that reduces this gap by producing large, rapid, and broad improvements in the mathematical competence of low-income preschoolers. Roughly, an hour of playing a simple, inexpensive, linear number board game produces gains in numerical magnitude comparison, number line estimation, counting, and numeral identification. Reasons for these large gains are discussed.