What you Need
What to do
Dissolve about 1 level teaspoon of borax in 2-300ml of warm water. - Borax powder is a strong cleaning agent and quite caustic, so get an adult to do this.
If you are using standard PVA glue pour about half a cup full into a container, and then dilute it with another half cupfull of water (if you want to end up with more goo, then use more glue!)
Add 1-2 teaspoons of borax solution and stir vigorously.
You may want to repeat the experiment with different amounts of borax.
After it starts to solidify, take it out of the container and knead it in your hands for a few minutes. - If you have sensitive skin you may want to wear gloves.
Have a play with it, how does it behave?
Wash your hands thoroughly after playing with it. Don't eat it, and don't put your fingers in your eyes when you are playing with it as any left over Borax is an irritant.
What may happen
You should find that the dilute PVA is very runny, but when you add the borax it will solidify and form a rubbery mass.
Why does it happen?
PVA glue is made up of PVA - poly vinyl alcohol molecules, in water. These are very long molecules which will tangle up with each other a bit when you stir the glue, making it quite thick and viscous, but there is nothing to stop it from flowing slowly.
The borax will form bonds to PVA molecules, and as each borax molecule can bond with more than one PVA molecule, it acts to stick them together. If there is just a little borax this will effectively make the PVA molecules longer, and it will go much more slimey. But if you add enough Borax it will form a great big interconnected network which will distort if you pull on it but will then wriggle back to its original shape, Molecular vibrations, a result of it being warm, cause each molecule it to become less straight. This means that it is stretchy like rubber and will bounce.
Why does the flubber flow very slowly?
The bonds between the borax and PVA are not very strong so occasionally they break and reform somewhere else. Each time this happens while the flubber is stretched it will flow slightly, and so it flows very slowly.