Pencils through a bag

What happens when you puncture a bag of water? …it leaks, right? Well here’s an experiment where we get pencils through a bag without losing a drop!

You will need

  • Sandwich or freezer size plastic bag
  • 10 sharp pencils
  • Water
  • Outside space


Note: It is a good idea to do this over a sink or in an outdoor space!

  1. Make sure all your pencils are nice and sharp before you start.
  2. Fill your bag half full of water and then seal the bag closed.
  3. Holding a pencil firmly in one hand and the bag in the other, carefully push the pencil through the plastic bag until it pierces the other side. Make sure the pencil stays in that position and you don’t push it all the way through. (You should see the pencil about a quarter length out of the plastic bag on both sides).
  4. Choosing another spot on the bag, away from the first, insert another pencil piercing both sides again.
  5. See if you can do all 10 without springing a leak!


  • Try different pencils. Some are round and some are more hexagonal in shape. Which shape pencil works best at sealing a potential leak?
  • Try different types of plastic bags you have at home. Some are thicker than others. Did this make a difference?
  • What happens when you take the pencils out?


The plastic bag you used was most likely a polymer made out of polyethylene. A very light weight, strong, flexible, long lasting material.

Molecules that make up polyethylene are like long strands of cooked spaghetti. When you push a sharp pencil through the polyethylene it slips between the flexible spaghetti strands, pushing them apart. Being flexible the spaghetti like molecules then tightly close around the pencil again making a temporary seal. However, when the pencil was removed the hole remained, as the initial over stretching of the polyethylene was permanent.

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