Lesson objective: Understand that the equal sign means that there are two equivalent quantities on each side of the equal sign.

Students bring prior knowledge of decomposing numbers less than 10 from K.OA.A.3. This prior knowledge is extended to numbers through 20 as students look for multiple solutions where two addends equal 20. Students also bring prior knowledge of comparing two two-digit numbers from 1.NBT.B.3. This prior knowledge is extended to understanding "the same as" or "the same value as" to mean "equal" and use the symbol (=) to convey that meaning. A conceptual challenge students may encounter is understanding that their solutions must total exactly 20. 

The concept is developed through work with a balance scale, which develops students' understanding of equality.

This work helps students deepen students' understanding of equivalence by requiring them to find multiple solutions to the same problem.

Students engage in Mathematical Practice 4 (model with mathematics) as they use balance scales to discover multiple ways to make a given total.

Key vocabulary:

• balance
• equal
• scale

Special materials needed:

• balance scale
• counters