This experiment is simple but will seem like magic. All you need is a plastic zip bag filled with water and sharpened pencils. Round are better and we will explain later.
All you have to do is take the sharpened pencil and poke it through the bag. You'll notice no water spills out. You can poke as many pencils as you want and the bag still remains full.
So why does this happen? The answer is something called polymers. The made is made up of polymers or chains of molecules that are flexible and give the bag its stretchiness. Think of these polymers as strands of freshly cooked spaghetti. When you push the pencil through the bag, it slips between and pushes apart the flexible polymers. The strands wrap around the pencil and form a temporary seal.
This is why round pencils are better. The polymers wrap around the round edges easier and create a better seal.
Just a warning though, when you take the pencils out, the polymers are permanently pushed aside and don't reseal so watch out.