The purpose of this activity is to create a ruler with centimeter units. Students begin their ruler at 0 and measure and label each centimeter up to 25 centimeters. It is important to include 0 on the ruler as this helps build the foundation for making sense of the number line in later lessons. Just as they will on the number line, students label each tick mark—not the space between the tick marks. They notice that the length between successive tick marks on their ruler is 1 centimeter and each tick mark represents a length in centimeters from 0, allowing them to use the ruler instead of counting cubes to measure (MP2).

This activity uses MLR8 Discussion Supports in the Synthesis to support students in describing how their ruler shows different lengths. Advances: speaking

• “¿Cómo pueden usar su regla para mostrarle a alguien qué tan largo es 1 centímetro?” // “How can you use your ruler to show someone how long 1 centimeter is?” (It’s the length from 0 to 1. It’s the length between any two numbers.)

MLR8 Discussion Supports

• If needed, invite students to repeat their reasoning using  mathematical language: “¿Pueden decir eso otra vez y usar la frase ‘puedo mostrarles la longitud desde __ hasta __’?” // “Can you say that again, using the phrase I can show them the length from __ to __?”
• “¿Cómo pueden usar la regla para mostrarle a alguien qué tan largo es 10 centímetros?” // “How can you use the ruler to show someone how long 10 centimeters is?” (It’s the length from 0 and 10.)
• “¿Qué otras longitudes podrían mostrar con su regla?” // “What other lengths could you use your ruler to show?” (lengths between 0 and 25 centimeters)
• “Ahora tenemos una herramienta para medir que podemos usar en vez de muchos cubos de un centímetro. En la siguiente actividad, van a usar las reglas que hicieron” // “Now we have a measuring tool we can use instead of using lots of centimeter cubes. You’re going to use the rulers you made in the next activity.”

The purpose of this activity is to measure the length of rectangles with the rulers created in a previous activity. If students’ rulers are not accurate, they should work with a partner who made an accurate ruler and encourage them to check their measurements with 10-centimeter tools and centimeter cubes. Students notice that each labeled tick mark on the ruler represents a length in centimeters from 0 (MP8). The Lesson Synthesis allows students to share their measurements for each rectangle. They discuss any different measurements that were made and what the source of the different measurements might be (MP6).

If students have measurements other than the precise measure of each rectangle, consider asking:

• “¿Cómo decidiste qué tan largo es el rectángulo?” // “How did you decide how long the rectangle is?”
• “¿En qué parte de la regla lo ves?” // “Where do you see this on the ruler?”

• Share measurements for each rectangle.
• Discuss any differences in measurement.
• “¿Cómo les ayudó el número 0 cuando midieron cada rectángulo?” // “How was the number 0 helpful when you measured each rectangle?” (It showed us where to put the tool. If you start with 0 then the length is the closest number to the end of the rectangle.)
• Invite previously identified students to share how they found the difference between the shortest and longest rectangles.
• “¿Cómo podemos usar nuestra regla para demostrar que el rectángulo más largo mide 10 cm más que el rectángulo más corto?” // “How can we use our ruler to prove that the longest rectangle is 10 cm longer than the shortest rectangle?”