Water Pressure

Water Pressure

Contributed by Yutao Zhou


If you puncture a bag full of water, will it leak? What if I told you that little, if any, water would escape?


  • One food storage bags (available in groceries store or online)
  • Two sharpened pencils
  • Be careful with the sharpened pencils! They are sharp. Only do this experiment under parent supervision.


  1. Fill up the food storage bags with water and seal the bag for convenience.
  2. Use one pencil to puncture at the upper area of the bag, and go through the entire bag. Try to keep it horizontal. (it works better if it only goes in the direction further in the bag)
  3. Use the other pencil puncture at the lower area of the bag, and go through the entire bag. Try to keep it horizontal.
  4. Observe whether there is water leaking out of the bag. (If the second and third steps have been followed strictly, there should not be any water leaking out)
  5. Pull out both pencils and observe the shapes of the water leaking out.

Physics Concepts and Questions

  • Why was there not any water leaking out of the bag when the pencils were in the bag?
    • The pencils inside the water bag blocked the hole, and water could not leak out. 
  • Why is the shape of the water leaking out what it is?
    • Water pressure: Pressure = Density * Height * g
    • The deeper the water is, the more pressure it would have. This is why water is leaking faster in the lower area of the bag and forms that shape.

Conclusions and Further Investigations

  • What if we use a hard water bottle instead of a soft bag?
  • What about atmospheric pressure?