Lesson objective: Count a collection two different ways.

Students bring prior knowledge of rote counting within 50 from K.CC.A.1. This prior knowledge is extended to counting within 100 by ones and tens as students practice counting larger collections of objects by ones and by tens. A conceptual challenge students may encounter is when counting by tens, we only say the decade numbers (such as 10, 20, 30, etc.) but each represents a quantity of ten more than the previous number said.

The concept is developed through work with counters and ten frames, which organize the objects into the structure of ten.

This work helps students deepen their understanding of number because in our base-ten number system, ones are organized in tens and multiples of ten. Our written numerals and most of our spoken numbers reflect this organization. As students develop their understanding of the sequence of ones, they also begin to make sense of how our written and spoken numbers are structured.

Students engage in Mathematical Practice 7 (Look for and make use of structure) as they notice that they can use the structure of ten to organize for counting. If each new set of counters has ten in it, it is more efficient to count the total number of objects by tens.

**Key vocabulary:**

- ones
- organize
- tens
- ten frames

**Special materials needed:**

- containers (for counters)
- counters
- journals/paper (for recording thinking-optional)
- ten frames