Activities & Days Out / 28 June, 2020 / Nanny Anita

Leak Proof Bag: Magic Trick or Science Experiment?

Don’t forget to follow our STEM board on Pinterest for more ideas.

Do you have a spare 5 minutes? Do you really want to impress some little ones with a cool trick?

You will need:

  • A zip lock bag
  • Sharpened pencils
  • Water

Fill your bag up with water and seal it.

Carefully push your sharpened pencils all the way through the bag, so that part of it is sticking out either side.

We had a go at this and both EC, YC and her friend loved it. They kept wanting to push more and more pencils through. EC, the cheeky monkey, also decided it would be more fun to start pulling the pencils out so that we would all get wet, much to YCs horror who had disappeared inside to get more pencils. Thankfully we have lots of zip lock bags so that we can do this trick over and over again.

EC pushing a pencil in, while deciding which one to pull out!

EC pushing a pencil in, while deciding which one to pull out!

The science behind it

Although this is a neat trick (especially when you do it over someone’s head), there is some pretty cool science behind it. The science might be too advanced for very young children but YC, who is now 5, understood the basic concept.

The zip lock bag is made up off a polymer called low-density polyethylene (LDPE). What is a polymer you say? It is a chemical material that is made up of the same substance bonded together, in this instance it is ethylene that has been polymerised.

One property of LDPE is that it is extremely flexible, because it is so flexible the tip of the sharpened pencil fits between the polymer strands by pushing them apart (imagine pushing your pencil through a helping of spaghetti). However, those strands then seal themselves around the pencil creating a barrier. When you then pull the pencil out, the hole remains and water leaks out because the polyethylene molecules have been pushed apart permanently.

Pretty cool if you ask me.

You can do further experiments with this to see what type of pencil works best; a many sided one or a round one, or you can try using different types of plastic bags to see if it will still work.

Whether you decided to do it as a trick or part of a science demonstration I highly recommend that you do it OUTSIDE, you know, just in case it doesn’t work.

Pin for later:

leak proof bag: is it just a cool magic trick or a neat science experiment? Either way it is sure to impress children.

In The Spotlight

Eaton Square Nursery Schools

At Eaton Square Nursery Schools, we offer a nurturing and stimulating environment for children aged 2 to 4 years old, with a rich and varied curriculum that encourages curiosity, creativity, and confidence. Our experienced and qualified staff are committed to supporting each child’s individual needs and interests. We have four nursery classrooms in two locations: Eccleston Square and Lupus Street. Eaton Square Nursery Schools children enjoy the benefit of priority admission to Eaton Square Prep School.



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