Check it out...
All you need is a ziploc bag, water, and some sharpened pencils (or wooden skewers, etc)
Once your bag is filled with water, simply push the sharpened end of the pencil straight through. If you do it fairly smoothly, the bag will seal itself around the pencil and no water will leak out. (However, I do recommend doing it over the sink or have a towel handy just in case.) We found that if you change direction midway through or start poking through the back and then hesitate, you might spring a leak. Most of the time, though, the pencil goes in one side and out the other without even a drip of water!!
It's lots of fun and we had a good time seeing just how many pencils we could put through before we got a leak.
There's some science behind this little trick. We learned about the polymers in the plastic bag stretching without breaking here on Steve Spangler Science, but it was really just about the fun.
You can check out the fun in action in this little video...
You can see we did have a little leak at the end. As we were rushing to get the bag over the sink, I said we may have done it too fast. Instead, I think the last pencil came too close to one of the other holes and the plastic in that section just wasn't strong enough to seal around the hole. Still fun, though!