Pre-K: Comparing Length

Warm up with a quick recall on the board. Compare objects directly (line them up at one end) and by measuring with informal units, blocks or paperclips placed end to end with no gaps, then compare the counts.

Before rulers, children compare and measure length directly and with informal units. They line objects up to see which is longer or shorter, then measure by laying identical units (paper clips, blocks) end to end and counting them. The key rules are a fair start line and no gaps or overlaps between the units. Model the method clearly, thinking aloud:

• Compare two objects directly first: line up one end and see which reaches further, teaching longer, shorter and the same.
• Introduce informal units (paper clips, cubes) and lay them end to end with no gaps and no overlaps.
• Insist every unit is identical, because mixing big and small units gives a meaningless count.
• Always start measuring from the very end of the object, giving a fair start line.
• Count the units to give the length (the pencil is 6 cubes long), then compare two lengths by their counts.

Do the first few questions of the practice worksheet together, one child explaining each step. Check for understanding before releasing the class to work alone.

Students complete the worksheet independently. Hand out the three difficulty levels below so every child works at the right stretch.

Circulate and look for these, they are the usual sticking points:

• Not lining up the starting ends, so the comparison is unfair.
• Leaving gaps or overlapping the units while measuring.
• Using units of different sizes and still counting them the same.
• Starting the count from one instead of the first whole unit.
• Not starting both objects from the same line, and leaving gaps or overlaps between informal units so the count is off.

Pull the class back together. Ask one child to explain comparing length in their own words, pose a single check question everyone answers on a mini whiteboard, and name what you will build on next lesson.

Use the independent worksheet as the evidence. A child who can complete it accurately and explain one answer has met the objective; anyone who cannot needs the easier level and a short reteach next session.