Teacher cards / Activity 05
For teachers and helpers. Adult supervised. Not for student screens.Leak-Proof Bag
Why does a bag of water not leak when a sharp pencil goes through it?
★The card
Put this card on the board for the class, or print it. Save the card
1Before class
The day before
- Test one bag yourself over a tub. It takes three minutes and shows you which bags work.
- Buy thick freezer zipper bags, quart size. Thin sandwich bags tear and leak. Test a few before class.
- Sharpen the pencils at home. Count them into a closed box. Count them out again at the end.
- Decide who pushes the pencils. Only the K to 5 teacher do it. Helpers and children never touch a sharp pencil.
- Give your helpers the helper guide on this page. Let them practice filling and sealing a bag. They do not practice with pencils.
- Check the floor and tables. Water finds the one spot you forgot. Have extra towels ready.
Ten minutes before kids arrive
- Set five tubs or trays on tables, one for each group. A dish tub works well.
- Put a folded towel under each tub and a second towel beside it.
- Pour about three cups of room temperature water into one pitcher for each group.
- At each group set two zipper bags, three crayons and three sheets of paper.
- Keep the sharp pencils in the closed box on the adult table. They never go to the groups.
- Put a trash bag by the door for wet bags.
- Put this card up on the board.
Safety and care
- An adult stays with every group for the whole activity. The K to 5 teacher is the responsible adult in the room.
- Only the K to 5 teacher touches a sharp pencil. Helpers and children do not hold, pass or push one.
- The adult pushes the pencil away from their own hand and away from every child. Points never face a person.
- The bag is always held over the tub. Children sit back at an arm's length with hands in their laps during every pencil turn.
- Pencils live in a closed box. Count them out before class, after each group and at the end. A missing pencil means stop and look.
- Plastic bags are a suffocation risk for young children. Keep bags off heads and faces.
- Nothing is tasted or drunk. The water is not for drinking.
- Spilled water makes a slip hazard. Wipe the floor at once and keep walkways dry.
- Wash hands before and after.
2Materials for 15 students
| Item | Per group | Whole class | Notes |
|---|---|---|---|
| Zipper freezer bags, quart size | 2 | 10 | Thick bags work best. One is a spare. |
| Water, room temperature | 3 cups | 15 cups | Two cups go in the bag. The rest is for spills and refills. |
| Pitcher or measuring jug | 1 | 5 | A helper pours with the child. |
| Sharp pencils, new and sharpened | 4 | 20 | Closed box. Only the K to 5 teacher touches them. Count in and out. |
| Dish tub or tray with sides | 1 | 5 | Holds the bag and catches the water. |
| Towels | 2 | 10 | One under the tub. One for hands and spills. |
| Crayons | 3 | 15 | For drawing. Not pencils, so no one mixes them up. |
| Plain paper, folded in half | 3 | 15 | Each child records what they see. |
| Closed box for pencils | 1 | A lidded box that stays shut between turns. | |
| Trash bag | 1 | For wet bags at clean up. |
How to set up the room
- Put the five tubs on tables or on the floor with towels under them. Leave two meters between tables.
- Put a strip of tape on the floor an arm's length back from each tub. Children sit behind the tape during pencil turns.
- Keep the adult table with the pencil box at the side of the room, away from the groups.
- Keep walking space clear and floors dry.
3Run it in 30 minutes
Groups sit at their tubs. Show the board. Ask for a guess. What will happen when a sharp pencil goes through a bag of water?
Each child helps pour. Helpers hold the bag open and press the zipper shut. Hold the bag over the tub and look for drips.
The K to 5 teacher visits each group in turn. Each turn takes about four minutes. The adult pushes the pencils and the children watch from behind the tape. Groups that wait draw the bag and what they think will happen.
Bring the class together. Compare what each group saw. Ask why the water stayed in.
Pencils go back in the box and get counted. Empty the bags into the tubs. Dry the tables.
4What the kids do
- Your helper holds the bag open over the tub. You pour in water until it is about halfway.
- Your helper presses the zipper shut along the whole line.
- Hold the bag over the tub and look. Are there any drips? Seal it again if there are.
- The adult pushes one pencil straight through both sides of the bag. Sit back at the tape and watch.
- Look closely. Is water coming out? Where does it come from?
- The adult adds more pencils, one at a time. Count the drips each time.
- The adult pulls one pencil out over the tub. What happens to the water now?
Match it to the grade
Watch the adult push the pencils. Count the pencils together. Say leak or no leak. Draw the bag with the pencils inside.
Count the drips after each pencil. Fill in a simple table. Say what changed when more pencils went in.
Compare a thick bag and a thin bag. Count drips for one minute each. Explain why the plastic hugs the pencil and what happens when it comes out.
5Helper guide for older students
Your job
- Lead one group of three. Practice filling and sealing a bag the day before.
- Hold the bag open while the child pours. Press the zipper shut with them.
- Give each child a job. One pours. One counts the pencils. One watches the drips.
- Ask questions and let the children guess before you explain.
- Keep the group behind the tape and keep the towels ready.
Say things like this
- What do you think will happen? Why?
- Look closely. Where is the water now?
- Let us count the drips together.
- That is a good guess. How could we test it?
Please do not
- Do not touch, hold or pass a sharp pencil. Not even to help.
- Do not hold the bag while a pencil is pushed through. The adult does that.
- Do not let anyone lean close to the pencil ends.
- Do not let a bag near anyone's face.
- Do not say a guess is wrong. Say let us test it.
6Talk it over
Questions to ask
- What did you think would happen when the pencil went in? Why?
- What did you see when the pencil went through the bag?
- What happened to the water as we added more pencils?
- What happened when the pencil came out? Why do you think so?
- Where have you seen a stretchy material squeeze around something?
- Grades 4 and 5. Why might a thin bag leak where a thick bag does not?
The science, in plain words
A zipper bag is made of polyethylene. It is a polymer, which means it is built from very long chains of repeating units. In a thin film the chains can slide past each other, so the plastic stretches. A sharp pencil pushes the plastic aside and stretches it more than it tears it. The stretched plastic then presses snugly all around the pencil. There is very little gap, so very little water gets out. A few drops are normal. Pull the pencil out and the hole stays open, so water flows. It works best with a thick bag, a sharp point and one smooth push straight through. Wiggling the pencil opens a gap and the bag leaks.
Where you see it
- Some tires have a soft sealing layer that hugs a nail so the air stays in.
- Plastic wrap stretches and clings around a bowl.
- A rubber band stretches and snaps back because its chains are long and flexible.
7If it goes wrong
| What you see | What to do |
|---|---|
| The bag drips before any pencil goes in | Press the zipper shut again along the whole line. If it still drips, swap in a spare bag. |
| Water streams out around the pencil | The bag may be too thin or the pencil may have wiggled. Use a thick freezer bag and push in one smooth motion. |
| The pencil will not go through | Check that the tip is sharp. The adult pushes straight and slow. Never push hard toward a hand. |
| The bag tears | Use a fresh thick bag. Push through the middle of the bag, away from the seams and the zipper. |
| Water spills on the table or floor | Stop and wipe it up at once with the towels. Check that the tub is under the bag. |
| Children crowd the tub | Point to the tape on the floor. Ask everyone to sit back with their hands in their laps. |
Clean up
- The adult pulls each pencil over the tub and puts it in the closed box
- Count the pencils back in. There should be 20
- Empty the bags into the tubs and pour the water into the sink
- Dry the tubs, tables and floor. Hang the towels to dry
- Put wet bags in the trash bag
- Wash hands
Go further
- Compare a thick freezer bag with a thin sandwich bag. Which holds better?
- The adult pushes a pencil above the water line. Does it matter where the pencil goes?
- Try a bag that is only a quarter full. Does it leak the same?
- Count the drips in one minute with one pencil and again with four.
Built by Science Fenns for teachers and student helpers. Keep supervising every group the whole time.