 Silly Putty and most other retail putty products have viscoelastic agents added to reduce the flow and enable the putty to hold its shape.The original coral-colored Silly Putty is composed of 65% dimethylsiloxane (hydroxy-terminated polymers with boric acid), 17% silica (crystalline quartz), 9% Thixatrol ST (castor oil derivative), 4% polydimethylsiloxane, 1% decamethyl cyclopentasiloxane, 1% glycerine, and 1% titanium dioxide.Silly Putty's unusual flow characteristics are due to the ingredient polydimethylsiloxane (PDMS), a viscoelastic substance.