Young children spontaneously engage in a variety of one-to-one correspondence activities during play. The present study tested whether one of these activities--pairing objects with containers--supported the development of numerical equivalence judgments. Three-year-olds were given sets of toys to take home. In one condition, the toys were accompanied by a container that facilitated one-to-one matching (e.g., wiffle balls and a muffin tin). In the other condition, the same toys were accompanied by a numerically equivalent set of loose objects (e.g., wiffle balls and plastic frogs). At test, children who had played with the containers showed significant improvement on a challenging number-matching task (i.e., cross mapping). In contrast, children who had played with the loose sets of objects continued to perform at chance. (Contains 3 figures, 2 tables, and 1 footnote.)