Sign up
3RD GRADE SCIENCE • ENGINEERING DESIGN

Comparing Solutions — Compare solutions using criteria.

Learn how engineers pick the best solution by testing ideas against a checklist of important rules.

SECTION 1

Why Do We Compare Solutions?

Have you ever had to choose between two or three ideas? Maybe you were picking a game to play at recess, or deciding what to build with blocks. People have been making choices like this for thousands of years! Engineers are people who design and build things to solve problems. They always have to compare different ideas before they pick the best one.

Long ago, people compared solutions just by guessing which one felt right. But over time, smart thinkers learned that using a list of rules — called criteria — makes choosing much easier and fairer. Let's look at how this idea grew over time.

3000 BCE
Ancient Builders
People in Egypt compared building materials like mud, stone, and wood to decide which was strongest for pyramids.
1400s
Leonardo da Vinci
This famous inventor drew many different designs for flying machines and bridges. He compared them to find the best one.
1800s
The Industrial Age
Factories began using checklists to compare products. They checked for cost, safety, and how well things worked.
Today
Modern Engineering
Engineers, scientists, and even students use criteria charts to compare solutions fairly and pick the best design.

So here is the big question: How do we compare different solutions in a way that is fair and helps us pick the very best one? That is exactly what this lesson is all about!

SECTION 2

Core Ideas About Comparing Solutions

Before we start comparing, we need to understand a few important ideas. These are the building blocks that help engineers — and you — make smart choices.

1

What Is a Solution?

A solution is an idea or design that tries to fix a problem. There can be many different solutions to the same problem.
2

What Are Criteria?

Criteria are the rules or standards you use to judge each solution. Think of them as a checklist of what matters most.
3

What Are Constraints?

Constraints are the limits you have to work with, like how much money or time you have. They help narrow down your choices.
4

Fair Comparison

A fair comparison means you test every solution using the same criteria. You don't skip rules for one idea and keep them for another.
✦ KEY TAKEAWAY
Think of criteria like the rules of a talent show. Every performer is judged on the same things — singing, dancing, and stage presence. If the judges used different rules for each person, it would not be fair! Criteria keep the comparison fair so you can truly find the best solution.
SECTION 3

See How Comparing Works

Let's look at a picture that shows how comparing solutions works. Imagine your class needs to build a bridge out of craft supplies. You have three different designs. The diagram below shows how you would check each design against your criteria.

This diagram shows three bridge designs at the top. Each one flows down through the same criteria checklist in the middle. The design that passes the most checks becomes the best solution at the bottom.

Notice how every design goes through the same three checks. That is what makes the comparison fair. You don't change the rules for each design. You use the same criteria every time.

SECTION 4

How Comparing Solutions Works Step by Step

Comparing solutions is a process. You follow steps, just like following a recipe. Here are the steps engineers use, and you can use them too!

The 4-Step Comparison Process

  1. Step 1 — Identify the Problem. What are you trying to fix or build? Write it down clearly.
  2. Step 2 — List Your Criteria and Constraints. Decide what matters most. Does it need to be strong? Cheap? Easy to make? Also think about limits like time and materials.
  3. Step 3 — Test Each Solution Against the Criteria. Go through each idea one at a time. Check how well it meets each rule on your list.
  4. Step 4 — Pick the Best Solution. Look at your results. The solution that meets the most criteria — or meets the most important ones — is usually the best choice.
💡 Important Tip!
Sometimes no solution is perfect. That is okay! Engineers often improve their designs after comparing. They take the best parts of different solutions and combine them.

You do not need fancy math for this. Instead, you use a comparison chart. A comparison chart is a table where you list your solutions in rows and your criteria in columns. Then you fill in how well each solution meets each rule.

SECTION 5

Different Kinds of Criteria

Not all criteria are the same. Some are about how well something works. Others are about cost, safety, or how easy it is to build. Let's look at the different kinds of criteria engineers think about.

This diagram shows five types of criteria that surround a problem. Engineers think about how well it works, cost, safety, ease of building, and impact on the environment.

When you set up your criteria, you get to decide which types matter most for your problem. If you are building a container to carry eggs safely, then safety (keeping the egg from breaking) might be the most important. If you are designing something to sell at a school fundraiser, cost might matter more.

SECTION 6

Worked Example: Choosing the Best Rain Shelter

Let's walk through a real example together. Imagine your class wants to build a small shelter to keep books dry during outdoor reading time. You have three design ideas. Let's compare them!

Comparing Rain Shelter Designs
1
Step 1 — Identify the ProblemBooks get wet when it rains during outdoor reading time. We need a small shelter to keep them dry.
Problem: Books get wet in the rain.
2
Step 2 — List Criteria and ConstraintsOur criteria are: (1) It must keep books dry. (2) It should be easy for kids to carry. (3) It should cost less than $5. Our constraint is that we can only use materials from the classroom supply closet.
3 criteria chosen: keeps dry, easy to carry, low cost.
3
Step 3 — Test Each SolutionDesign A — Plastic Bag Cover: Keeps books dry ✅. Very easy to carry ✅. Costs about $1 ✅. Design B — Cardboard Box: Does NOT keep books dry because cardboard soaks up water ❌. Easy to carry ✅. Costs about $2 ✅. Design C — Folder with Tape: Keeps some water out, but not all of it ⚠️. Easy to carry ✅. Costs about $3 ✅.
Design A meets all 3 criteria. Design B meets 2. Design C meets about 2.5.
4
Step 4 — Pick the Best SolutionDesign A — the plastic bag cover — meets all three criteria. It keeps books dry, it is easy to carry, and it costs the least. Design A is the best solution!
Winner: Design A — Plastic Bag Cover 🏆
Comparison chart for three rain shelter designs
DesignKeeps Dry?Easy to Carry?Under $5?Score
A — Plastic Bag✅ Yes✅ Yes✅ Yes ($1)3 / 3 ⭐
B — Cardboard Box❌ No✅ Yes✅ Yes ($2)2 / 3
C — Folder + Tape⚠️ Partly✅ Yes✅ Yes ($3)2.5 / 3
SECTION 7

Strengths and Limits of Comparing Solutions

Using criteria to compare solutions is a great tool, but like any tool, it has strengths and limits. Let's look at both sides.

Strengths ✅Limits ⚠️
Makes choices fair — everyone uses the same rules.You might forget to include an important criterion.
Helps you organize your thinking clearly.Some criteria are hard to measure, like "looks nice."
You can explain your choice to others using evidence.Not all criteria are equally important, so a simple count might not tell the whole story.
Works for all kinds of problems — big and small.Sometimes you need to test and rebuild before you can really compare.
✦ KEY TAKEAWAY
Using criteria to compare solutions is like using a scorecard at a gymnastics meet. The scorecard does not do the gymnastics for you, but it helps the judges make a fair decision based on evidence, not just feelings. The more carefully you choose your criteria, the better your final choice will be!
SECTION 8

From Comparing to Improving Designs

Comparing solutions is an important step, but it is not the last step! Real engineers use what they learn from comparing to make their designs even better. This is called improving or optimizing a design. Let's see how comparing connects to what comes next.

Comparing Solutions (What You Learned Today)Improving Designs (What Comes Next)
You look at multiple solutions side by side.You take the best solution and make it even better.
You use criteria to judge each one.You test and redesign based on what didn't work well.
You pick a winner from the group.You keep testing until the winner is as good as it can be.
You explain why one solution is best.You share your improved design with others for feedback.

In the next part of engineering design, you will learn how to test, improve, and redesign your best solution. Comparing is just the beginning of the engineering journey!

SECTION 9

Practice Problems

PROBLEM 1 — CONCEPTUAL
What are criteria? Explain in your own words why they are important when comparing solutions.
PROBLEM 2 — BASIC CALCULATION
Your class is comparing two designs for a pencil holder. Design X meets 2 out of 4 criteria. Design Y meets 3 out of 4 criteria. Which design is the better choice based on the criteria? Why?
PROBLEM 3 — INTERMEDIATE
You are designing a container to carry a raw egg without it breaking. Your criteria are: (1) the egg stays safe, (2) the container weighs less than 200 grams, and (3) it can be built in 15 minutes. Design A uses bubble wrap and a box — the egg stays safe, it weighs 180 grams, but it takes 25 minutes to build. Design B uses cotton balls in a bag — the egg stays safe, it weighs 100 grams, and it takes 10 minutes. Which is the better solution? Explain.
PROBLEM 4 — APPLIED
Your school wants to reduce litter on the playground. Three solutions are suggested: (A) add more trash cans, (B) assign students as "litter patrol" helpers, and (C) put up posters asking people not to litter. Think of three criteria you would use to compare these solutions. Then explain which solution you think might be best and why.
PROBLEM 5 — CRITICAL THINKING
Two students compare designs for a birdhouse. Student 1 says Design A is better because it is prettier. Student 2 says Design B is better because it keeps rain out and is the right size for birds. Who is using criteria the right way? What advice would you give to Student 1 to make a better comparison?
SUMMARY

Lesson Summary

In this lesson, you learned that engineers and scientists compare solutions to find the best way to solve a problem. Instead of just guessing, they use criteria — a set of rules or standards — to judge each idea fairly. They also think about constraints, which are limits like time, money, and materials. Different types of criteria include how well it works, cost, safety, and ease of building.

The four steps to compare solutions are: identify the problem, list your criteria and constraints, test each solution against the criteria, and pick the best solution. A comparison chart is a helpful tool for organizing your thinking. Remember, comparing is just one part of the engineering design process — next, you will learn to improve and redesign!

Varsity Tutors • 3rd Grade Science • Comparing Solutions — Compare solutions using criteria.