What is silly putty? Chemical & Engineering News: What's That Stuff? explains it:
"Silly putty is a dilatant compounds, which means it has an inverse thixotropy--that is, as a viscous suspension or gel, it becomes solid under the influence of pressure."
The manufacturer of silly putty, Binney & Smith, noted that it has "been the subject of dissertations by aspiring physicists and chemists."
OK, so what is silly putty? It's a mixture of silicone oil with boric acid.