How many sharp wooden skewers can your child poke through a plastic bag filled with water before it leaks?
You may be surprised!
For this astonishing experiment, you’ll need:
- A resealable plastic bag, like a Ziploc baggie (thick freezer bags work best)
- Food coloring
- Sharp wooden skewers
- Sharpened pencils (optional)
Here's how to make — and test — the magic bag:
- Have your child fill a plastic bag about 2/3 of the way full with water.
- Let them add a drop or two of food coloring to the water.
- Seal the bag shut. Ask your child to guess what would happen if they poked a sharp stick through the bag.
- Go outside, or hold the bag over a tray or sink. Ask your child to push the skewer all the way through the part of the bag filled with water.
- Let them see how many skewers they can push through before the bag starts to leak.
- Try the same experiment using sharp pencils.
Safety hint: Supervise small children to prevent injuries from the sharp ends of wooden skewers.
The Science Behind the Fun
“Poly” means many, and “mer” means parts, so the word polymer means “many parts.” Plastics are polymers (long chains of molecules, like beads on a string). Some polymers, like the ones in plastic bags, are good at stretching and forming seals.
When you poke a skewer through the bag, the plastic forms a seal so it doesn’t leak. The forces of surface tension are also at work and help to keep the bag from dripping where you’ve pierced it, since water molecules really like to stick together.
Want to see how this experiment plays out? Watch this video demo!