Keeping the Test Fair
A 55-minute Grade 3 lesson on planning fair tests that control the variables and look for the points where a model fails.
Keeping the Test Fair is a free 55-minute Grade 3 to Grade 5 science lesson plan for Common Core standard 3-5-ETS1-3 — run fair tests, controlling the variables, to find what to improve. It is timed across 5 sections, gives the exact wording to use where the wording carries the mathematics, names 3 misconceptions with the teaching move that addresses each, and ends with an exit ticket and a printable worksheet with its answer key.
- Grade:
- Grade 3 to Grade 5
- Length:
- 55 minutes
- Standard:
- 3-5-ETS1-3
Grade 3 to Grade 5 · Engineering Design
3-5-ETS1-3
Plan and carry out fair tests in which variables are controlled and failure points are considered to identify aspects of a model or prototype that can be improved.
Objective
Students will be able to plan a fair test of a prototype and name its failure point.
Students are successful when they can
- Names the one variable being changed.
- Names what is held the same.
- Identifies where the prototype failed first.
What you need
- Model bridges or towers built from card and straws
- Weights, and a record sheet with a controlled-variables line
- The printable practice sheet linked at the end of this plan
The lesson
Warm-up
5 min- Load two bridges to failure, one tested on a table and one on a cushion.
- Ask whether the comparison means anything.
Teach — control, then watch for the failure
14 min- List what has to be held the same: span, support, how the weight is added.
- Name the one thing being changed: the design.
- Load the first bridge slowly and note WHERE it gives way, not just that it did.
- Say the term: that is the failure point, and it is what to improve.
- Repeat with the second and compare failure points.
- Say why recording controls matters: somebody else has to be able to judge the test.
“Where did it give way first? That is what you improve.”
Investigate — test to failure
19 min- Groups load their prototype gradually, adding weight in equal steps.
- They record the load at failure and the point that gave way.
- They repeat with one change to that point only.
- They compare the two loads and say whether the change helped.
Independent practice
14 min- Students complete the 'Keeping the Test Fair' page.
- Check the answer names controlled variables.
Close
3 min- Ask which failure point was most common across the class.
What goes wrong, and why
Each of these is a reasoning error rather than carelessness. The fix addresses the reasoning.
Changes two things between tests.
Why: Both looked improvable.
Fix: Then you cannot say which helped. One at a time.
Records only that it broke.
Why: Breaking is the event.
Fix: Where it broke is the useful part. That is what you redesign.
Tests once and redesigns.
Why: One test showed the weakness.
Fix: Repeat it. A single failure might be a bad join rather than a bad design.
If they are not there yet
- A prototype supplied, with the test to run.
- A record sheet listing the controls to tick.
If they finish early
- Improve the failure point and predict the new load.
- Find a design whose failure point moves after improvement.
- Say what test would be unfair however careful you were.
Exit ticket
You test two bridges. Name the variable you change, two things you hold the same, and what a failure point is.
What to look for: The design changed; span and loading held the same; the failure point is where it gives way first. Missing controls means the fair-test idea needs another run.
Printable practice for 3-5-ETS1-3
Free to print and copy, answer key included. Each sheet can generate a fresh set of problems for a retake.
- Keeping the Test FairCircle the best answer.
Questions about Keeping the Test Fair
How long does the Keeping the Test Fair lesson take?
55 minutes, split across 5 timed sections: warm-up 5 min, teach 14 min, investigate 19 min, independent practice 14 min, close 3 min. The minutes are stated per section and add up to the stated length, so the plan can be cut or extended at a section boundary rather than abandoned halfway.
Which standard does Keeping the Test Fair teach?
3-5-ETS1-3 — 3-5-ETS1-3, Engineering Design for Grade 3 to Grade 5: “Plan and carry out fair tests in which variables are controlled and failure points are considered to identify aspects of a model or prototype that can be improved.” In plain terms, run fair tests, controlling the variables, to find what to improve.
What do I need to teach this lesson?
Model bridges or towers built from card and straws, weights, and a record sheet with a controlled-variables line and the printable practice sheet linked at the end of this plan. Everything listed is either ordinary classroom equipment or a free printable from this site — there is nothing to buy and nothing to prepare beyond photocopying.
What mistakes should I expect in this lesson?
Three, each a reasoning error rather than carelessness: changes two things between tests; records only that it broke and tests once and redesigns. The plan gives the thinking behind each one and the teaching move that addresses the thinking rather than the symptom.
How do I know whether the lesson worked?
The exit ticket asks: “You test two bridges. Name the variable you change, two things you hold the same, and what a failure point is.” The design changed; span and loading held the same; the failure point is where it gives way first. Missing controls means the fair-test idea needs another run. That tells you whether the class needs a reteach or a thirty-second conversation before you plan tomorrow.
Is there a worksheet to go with Keeping the Test Fair?
Yes — 1 printable 3-5-ETS1-3 worksheet, linked at the end of the plan and free like the rest of the site. Each one comes with an answer key and 25 versions, so the practice after the lesson can be a different paper on every desk.
Is Keeping the Test Fair free to use?
Yes. The plan, its printables and their answer keys are free to read, print and photocopy for your own class, with no account and no email capture. It was last reviewed on 8 September 2026. Please do not resell it or republish it as your own.
Last reviewed 2026-09-08. Aligned to 3-5-ETS1-3 of the Next Generation Science Standards.