Many problem situations involve change—adding to or taking away from a set. Such changes lend themselves to concrete models or to acting out as sets are joined or separated and then counted to find out “how many now?”. However, it is also important for young children to make the more foundational generalization that adding increases and taking away decreases the quantity in a set.
© Erikson Institute’s Early Math Collaborative. Reprinted from Big Ideas of Early Mathematics: What Teachers of Young Children Need to Know (2014), Pearson Education.