I've seen this experiment on loads of blogs and for good reason. It's guaranteed to make kids' jaws drop. It seems more like magic than science.
Here's how we poked holes in a bag without it leaking.
Step 1: Sharpen pencils to a fine, sharp point.
Step 2: Fill up a clear zip-top plastic food storage bag so it is 2/3rds full of water and seal it closed.
Step 3: Hold the bag up and slowly insert pencils piercing the plastic, going through the water, and out the other side of the bag. Make sure you don't poke the eraser all the way through. Why stop with one? Keep putting pencils in!
Step 4: Be amazed that the bag isn't leaking.
Step 5. Move the bag to the sink to remove the pencils. Water will squirt out everywhere!
Invite kids to form a hypothesis about why this works. My boys thought the pencils were so tight that the bags didn't leak. They were close. The polymers in the plastic are chains of molecules that separate but don't break when poked with the pencil. They squeeze in so snugly around the pencil, it forms a tight waterproof seal.
This experiment came from the fabulous Steve Spangler.