Surprise! No Leak!

by Katie Brazerol

Here is a quick and easy trick you can do to amaze the children in your setting! Step-by-step picture tutorial and extension idea included.


Please note: Test the plastic zippered bag ahead of time to make sure there are no leaks in the seal. You may wish to try the experiment on your own before showing to children to make sure you know how much pressure to apply when pushing the pencil through the bag. (The first time I tried it, I didn’t push hard enough and I ended up poking a hole into the bag without the pencil going through.)

What You’ll Need:

  • plastic zippered bag
  • sharpened pencil
  • glass of water
  • sink or tub

Step 1: Gather your materials and fill the plastic zippered bag about half full with water.

Seal the bag. Hold up a sharpened pencil and ask children, “What do you think will happen if I poke this pencil through the bag of water?” Most children will probably reflect on how sharp objects will cause a leak if poked through a soft material, so they will answer that the water will begin to leak out of the bag.

Step 2: Hold the bag of water over a sink or a tub. Pick up the sharpened pencil and swiftly poke it through the bottom of the bag so about half the pencil pushes through the bag. Leave the pencil in place, and ask children to watch for drips or leaks. If done properly, the bag should be leak-free.

Ask children, “Why do you think the water didn’t leak when the pencil poked the bag?” Explain that the plastic bag was stretchy enough to form a seal around the pencil. This prevented any water from escaping.

Talk about how sailors can fix leaks in their boats. They need a substance that can fill the hole and create a seal. Sometimes a special putty-like material can be used to fix a small leak. If a boat is inflatable, a patch might be glued to the area. Ask children, “What do you think would happen if a leaky boat isn’t fixed?”

Extension: What happens when you poke more than one pencil in the bag? Can you poke a pencil through one side of the bag and back out the other without creating a leak in either side? Experiment with the children.

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