DOUBLE TAP TO ZOOM WITH PHONE OR TABLET What Is Mathematizing?   3 Figure i.1 and Figure i.2. Teachers Elvira Mata and Andrea Richard mathematizing children’s water explorations at the Laurance Armour Day School in Chicago, Illinois design, and application of rich mathematical experiences in the classroom. It develops students’ math understanding in a natural, yet rigorous process. To get a sense of how teachers can mathematize experiences within a school setting, let’s take a look at two examples from the Belmont-​Cragin Early Childhood Center, a public school in Chicago. Teachers engaged chil- dren and parents in meaningful math learning experiences as a way to en- courage mathematizing at school and at home. In teacher Lourdes Molina’s preschool classroom, children became curi- ous about a rainbow that appeared in the sky one rainy day. The class had a long discussion about rainbows, and Mrs. Molina decided to have the chil- dren investigate the topic more in-​depth through a long-​term study. At first, the children observed and focused on the colors and shapes of a rainbow. This interest led to a recycling project where the children were encouraged to collect items at home based on the colors they identified on the rainbow. Once the materials were collected, children grouped the materials based on color, counted the number of items per set, and created a color-​pattern rain- bow. Sometime later, as children revisited the rainbow project, Mrs. Molina challenged them to think about the length of the rainbow lines. She asked children to estimate the length of each line and to predict which lines would COPYRIGHTED MATERIAL