Interestingly enough, Silly Putty was invented/discovered accidentally during government-funded research to develop a rubber substitute, primarily for military purposes, during WWII. Since then Silly Putty has provided countless hours of joy for millions of kids. Proof that defense spending can sometimes make the world a more fun place

Oddly enough, Napalm has a similar story, and it twice as much fun. But as Napalm is not up this month, I'll vote for silly putty.