Silly Putty, a trademark of Crayola LLC, is made primarily from silicone and color pigments.
While credit for its creation is disputed and has been attributed variously to Earl Warrick, Harvey Chin and James Wright, Crayola honors the latter who is said to have discovered it by mixing boric acid and silicone oil together.
The original coral-colored Silly Putty is composed of:
65% dimethyl siloxane (hydroxy-terminated polymers with boric acid)
17% silica (crystalline quartz)
9% Thixatrol ST
4% polydimethylsiloxane
1% decamethyl cyclopentasiloxane
1% glycerine
1% titanium dioxide
The polymers in silly putty have covalent bonds within the molecules, but hydrogen bonds between the molecules. The hydrogen bonds are easily broken. When small amounts of stress are slowly applied to the putty, only a few bonds are broken and the putty "flows".
When larger amounts of stress are applied quickly, there are many hydrogen bonds that break, causing the putty to break or tear.
Of interest, see an infographic charting of the top 60 greatest inventions of all time from Raconteur.net.