What you need:
- a ziplock plastic bag is best, but any good plastic bag will do
- water
- pencils
What to do:
1.Fill the bag about three-quarters full with water, and zip or knot it shut
2.Poke a pencil right through the bag and leave it there
3.See if you can pull the pencil back out and put it in the same holes again

Watch awesome video clips
Luv Sarah
wat happens it has nothing to do with the shape or size of the bag its all to do with pressure and pencils wen you put in the pencil all the way throught to the other side the outside pressure forces on the inside pressuer and keeps an even force so the pencil blocks the water getting out
xoxo