Materials

• Zip-lock plastic bag

• Several sharp, round pencils

• Water

• Sink (for practice)

Instructions

Watch video instructions here (time 1:00)!

1. Sharpen your round pencils. 

2. Fill your bag one-half full with water and seal the bag closed. 

3. Hold a pencil in one hand and the top of the bag in the other hand. Push the pencil through one side of the bag and then the other. 

4. Repeat! 

helpful hints

• Make sure the pencils are round and the tips are super sharp. 

• Be careful not to push the pencils all the way through both of the holes. 

• Try it over a sink or bathtub to practice your skills!

cool fact

The long chains of flexible molecules that make up the bag magically seal back around the pencil and prevent water from leaking out. 

Why? Polymers! The bag you used was probably made out of a common polymer called low-density polyethylene (LDPE). Think of the LDPE molecules as strands of cooked spaghetti. The sharp point of the pencil can easily slip between and push apart the flexible strands of spaghetti. This flexibility helps to form a temporary seal against the edge of the pencil. When you remove a pencil, the hole in the bag stays because the LDPE molecules were pushed aside permanently and the water spills out. Enjoy playing with polymers!