Still, what did this have to do with flubber? How did the borax turn the watery glue into a silly putty like substance? Why did the flubber grow more solid as it was played with? We understood somewhat that the borax was forming a covalent bond with the polymer of glue and water, joining together the individual chains of molecules - but we were going to need to see it and touch it, to understand it.
Out came the gumdrops! We used fishing line to make two separate chains of gumdrops. We snaked them around the table, and dangled them in the air, observing how floppy and free moving they were. Then we connected the two chains together with toothpicks. Suddenly, we had a rigid track instead of two free moving chains.
The gumdrop chains represented our glue and water mixture. The toothpicks represented the borax. Apparently, the borax forms a fairly strong bond between the polymers, which allows the flubber to flow like slime, while the glue and water polymer chain is held together by a hydrogen bond which breaks when pressure is applied. This is why our flubber sometimes breaks into little pieces, or tears jaggedly in half.
My science knowledge for that last paragraph was pieced together from quite a few different sources, so don't take my word for it, but I think we're pretty close to understanding the way it works. When I saw the gumdrop chains on various websites, I was a little skeptical as to whether it was a worthwhile exercise, because it seemed so simple. But in this case, touching was believing. There was a real physical difference between the disconnected and connected chains, that really helped to bring the concept of covalent bonds home. Plus we got to eat the extra gumdrops.
It's great to be a homeschooler!