Fractions constitute a stumbling block in mathematics education. To improve children's understanding of fractions, we designed an intervention based on learning-by-doing activities, which focused on the representation of the magnitude of fractions. Participants were 292 Grade 4 and 5 children. Half of the classes received experimental instruction, while the other half pursued their usual lessons. For 10 weeks, they played five different games using cards representing fractions (e.g., Memory and Blackjack). Wooden disks helped them represent and manipulate fractions while playing games. Our results showed an improvement in the conceptual understanding of fractions. The findings confirmed that the usual practice in teaching fractions is largely based on procedural knowledge and provides only minimal opportunities for children to conceptualize the meaning and magnitude of fractional notations. Furthermore, our results demonstrate that a short intervention inducing children to manipulate, compare, and evaluate fractions improves their ability to associate fractional notations with numerical magnitude. (Contains 6 tables.)