Here's an amazing science trick that looks like magic - stick pencils right through a plastic bag filled with water... without any water leaking out! But do this activity outside because the water will spill when you pull the pencils out. The tricky part is pouring water into the zipped plastic bag, so ask someone if they can hold the bag open for you. You will need a clear zipped plastic bag, several coloured pencils and a pitcher of water. 1. Set the bag on a flat surface, then slowly and carefully fill it nearly to the top with water. Ask someone to hold the bag open for you if you need. 2. Seal the top of the bag tightly and securely, trying not to spill any water. 3. Holding the bag at the top with one hand, push a pencil, sharpened end first, through the bag in one smooth movement. 4. Stick all your pencils through the bag one by one. Still no water leaks out! Remember, go outside or hold the bag over a sink when you remove the pencils. HOW IT WORKS Plastic kitchen bags are made of polyethylene, a strong but flexible material. When the pencil makes a hole through it, the polyethylene wraps tightly around the pencil, but leaves a tiny gap - a hole through which water could escape. Water molecules cling together, so if a hole is small enough, the clinging force between the water molecules near the hole is strong. If it's stronger than the push from the water molecules inside, the water won't escape. Make the hole much bigger and the molecules can't hold back the pressure from inside, and the water leaks out.