The science club met in two groups to learn about polymers.  The glue contains a polymer called polyvinyl acetate resin.   Students changed the polymers  behaviour twice in this activity; Once when we added water and the second time when we added borax.   Adding the water was a physical change where as adding the borax results in a chemical change.     The borax is called a cross-linker.   It chemically "ties together" the long strands of the polyvinyl acetate.   This tying together changed the viscosity of the glue.  It increased the viscosity because of the new cross-linked chains interfere with the ability of the solution to flow.   As a result the silly putty is "stiffer".   It is not a solid thow..... If we leave the silly putty alone on a table it will flaten out!  Solids keep their shape. See Corresponding Photo Gallery under the Science Club Tab:  http://hms.nbed.nb.ca/club/gallery/making-polymer-silly-putty