Variables and Failure Points

All three teams had the exact same materials. So why did one bridge work so much better than the others? And what exactly caused each bridge to fail?

Scientists and engineers ask questions just like this every day. When something doesn't work, they need to figure out what changed between the designs and where the weakness was that caused it to break.

• What do you think was different about how each bridge was built?
• Where exactly on each bridge do you think it started to break?
• If you wanted to test why one bridge was stronger, what would you change — and what would you keep the same?

When engineers design something — like a bridge, a toy, or a building — they need to think carefully about what might make their design work or fail. To do this, they use two very important ideas: variables and failure points.

🔬 Investigation Spotlight: Testing Paper Bridge Variables

What scientists and engineers do: They plan and carry out investigations where they change one variable at a time to figure out what affects how well a design works.

Investigation question: How does the shape of a paper fold affect how many pennies a paper bridge can hold?

Materials you would need:

• 6 sheets of paper (all the same size)
• 2 stacks of books (same height, same distance apart)
• A cup of pennies
• A data table to record results

What you would do:

• Keep the same: paper size, book height, distance between books (these are controlled variables).
• Change ONE thing: the shape of the paper fold (flat, single fold, accordion fold — this is the variable you test).
• Test each design: Place pennies on the bridge one at a time. Record how many pennies it holds before it fails.
• Observe the failure point: Watch carefully — where exactly does the bridge start to bend, sag, or break?
• Run 2 trials for each fold shape. This helps you know if your results are reliable.

Notice step 5: engineers change one variable based on what they learned from the failure point — and then they test again! This cycle keeps going until the design works well enough.

When students ran this investigation, they collected data in a table. Look at the results below. The data tells a clear story about how one variable — the fold shape — made a big difference in how strong the bridge was.

The data shows that changing just one variable — the fold shape — made a huge difference. Bridge C held almost 8 times more weight than Bridge A! This happened because the accordion folds spread out the weight across many ridges. The flat paper had no support at all, so it sagged right away.

Each bridge had a different failure point. Bridge A failed in the center because flat paper bends easily under weight. Bridge B's single fold was stronger, but the fold itself eventually flattened out. Bridge C lasted the longest, but even it failed when the edges of the accordion folds finally buckled under the heavy load of pennies.

By studying these failure points, an engineer could now ask: "How can I fix the edge-buckling problem on Bridge C?" Maybe reinforcing the edges or adding tape would help. That's the next variable to test!

The crosscutting concept we are exploring in this lesson is Cause and Effect. Scientists and engineers know that events have causes that generate observable patterns. When engineers design tests to identify causes, they change one variable at a time.

This pattern — changing one thing and watching what happens — shows up everywhere in science. Let's look at how cause and effect works across different areas of science:

Do you see the pattern? In every example, there is one cause (the variable that changes) and one effect (the result we observe). When we control all the other variables, we can clearly see what caused the result. This is how engineers identify failure points — they figure out which specific cause led to the failure.

Engineers in the real world use variables and failure points every single day. When a real bridge, building, car, or airplane is designed, engineers go through the same process you learned about — just on a much bigger scale.

Here is how the engineering design process works when engineers build something like a new playground structure:

Real engineers also study failures from the past. When a bridge collapses or a phone screen cracks, teams of engineers investigate the failure point. They ask: "What variable caused this? Was the material too weak? Was the design the wrong shape? Was there too much weight?" This careful detective work leads to safer, better designs for everyone.

You are already thinking like an engineer when you build something, watch it break, and think about what to change. Every time you say "Let me try it a different way," you are using the engineering design process!

Variable — Anything in a design or experiment that can be changed. In a paper bridge test, the fold shape, paper size, and number of pennies are all variables.

Failure Point — The specific spot or reason where a design breaks, bends, collapses, or stops working the way it should.

Fair Test — An investigation where you change only one variable at a time and keep everything else the same, so you know exactly what caused the result.

Controlled Variable — A variable that you keep the same during a test so it doesn't affect the results. Also called "things you keep the same."

Engineering Design Process — The step-by-step method engineers use to define a problem, design a solution, test it, find failure points, and improve the design.

Cause and Effect — The relationship between something that happens (the cause) and what it leads to (the effect). Engineers use fair tests to identify cause and effect.

Trial — One round of testing. Running multiple trials helps you know if your results are reliable.