Sorting starts with a property
To sort things you need a question with a yes or no answer. Each one splits the pile in two.
property Something you can say about an object, like hard, soft, red, or see-through.
Step 1: Something to Notice
Watch first. The explanation comes later.
Sort the same five materials twice, by two different properties, and glass ends up in a different pile each time.
The glass did not change. So why did its group?
Step 2: Find Out
Read down a single column. That column is one sort.
Step 1 — Predict
If you sort by "hard", will glass be in the same group as metal?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 3: So Here Is Why
Now the explanation, after you have seen it happen.
How to sort
Pick one property and only one. Ask that same question about every object.
Sorting by hard
Wood, metal and glass are hard. Cloth and rubber are not.
Now sort the same things by see-through
Glass is see-through. Wood, metal, cloth and rubber are not, so they go together.
Same things, different piles
Sort by a different property and different piles come out. The piles show what you asked.
Step 4: A Common Mistake
Lots of people think
“Things belong in one group. There is a right answer to which pile something goes in.”
Step 5: Sorting Anything
The same idea somewhere new.
You could sort your class by height, or by age, or by who has a pet. Same people, three completely different sets of groups.
Step 6: Think It Through
Practice makes it stick.
Two Piles
Problem 1 of 1
You sort by "keeps water out". Which of these goes in the Yes pile?
Sorting
1 of 3
Sorted by "see through", which one is on its own?
2 of 3
Does a material always go in the same group?
3 of 3
Sort these by "keeps water out".
Tap something to move it.
- Empty
- Empty
Step 7: Quick Check
Show what you know.
Question 1 of 1
You change which property you sort by. What happens to the groups?
What You Found Out
- To sort, pick one property.
- Different properties give different groups.
- There is no single right pile.
For the grown-up
The word doing the work here is "property", and it is worth saying out loud a few times with real objects in hand. A child who has not pinned that word down cannot follow anything else in the lesson.
Sort on one property at a time. Trying to sort by color and stiffness and weight at once produces a muddle, and the muddle is what makes children think sorting has a right answer they are failing to find.
There is no single correct sorting. That is the point of the lesson and it is genuinely surprising to a seven-year-old, who usually expects one right answer. If they ask which grouping is the correct one, the honest reply is that both are.
A real box of objects beats the screen here. Buttons, a spoon, a sock, a glass, a wooden block — sort them twice on the table and the idea lands in a way no diagram manages.
A question with two answers — To sort things you first need a question with a yes or no answer. Is it hard? Is it red? Is it see-through? Each of those is a property, and each one splits your pile into two: the things that have it and the things that do not.
How to sort — Pick one property and only one. Go through the objects one at a time and ask that same question about each. Yes goes in this pile, no goes in that pile. Changing the question halfway is what makes sorting go wrong.
Sorting by hard — Wood, metal and glass are hard, so they go together. Cloth and rubber are not hard, so they go in the other pile.
Now sort the same things by see-through — Glass is see-through, so glass goes on its own. Wood, metal, cloth and rubber are not see-through, so now they are all together.
Same things, different piles — Sort the same objects by a different property and completely different piles come out. Nothing about the objects changed — glass is still glass. The piles show what you asked, not what the things really are. So the useful question is always: which property matters here?