You need: A polyethylene bag, some ordinary pencils, water.
The experiment: Fill about half the polyethylene with water. Pierce the bag using the pencils in the places where it's filled with water.
Explanation: If you pierce the bag with the pencils and only then pour in the water, it will seep out through the holes you've made. But if you fill it first and then insert the pencils, the water will not seep out at all. This is a result of the fact that when the polyethylene is broken apart its molecules move closer together. In this case, the polyethylene tightens around the pencils.