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Why do we use polythene for packing materials?
The zipper lock bag you used was likely made of a material low density polythene (LDPE). This polymer is one of the most widely used packing materials in the world. And for good reason! LDPE is lightweight, durable, a barrier to moisture, low in cost and very flexible.
We can imagine these polythene molecules as strands of freshly cooked spaghetti. The tip of the pencil can easily push the strands of spaghetti apart, but the flexibility of the strands helps to form a temporary seal around the edge of the pencil. When we remove the pencil, the holes remain in the bag because polythene molecules were pushed aside permanently and the water leaks out.