We filled the balloon, using a hair dryer, and then let it rise. Our first results were not very good. The thread seemed to weigh the balloon down, and the open mouth allowed the cool wind in, reducing the lift. Finally, we decided to tie the balloon closed. This worked much better.
At first we weren't getting much more than 10-20 feet off the ground. The breeze would take the balloon out over neighbor's yards, and we would pull it back, just barely clearing our fence. We had one tense moment, when I thought the balloon was going to be stuck on our next door neighbors roof. Happily, we were able to pull it back, our thread proving stronger than I thought it would. I'm not sure if the neighbor noticed the large black pyramid drifting past his patio door or not. Maybe we'll take him a plate of cookies just in case.
Then, the breeze died down, and the balloon rose straight up. It just kept going and going. It was really marvelous. I grabbed the camera to snap a picture or two, instructing my son to watch for the end of the thread on spool. Of course, I had barely gotten my instruction out, when he yelled, "Oh, no!"
You can probably guess the rest.
It's great to be a homeschooler!