Stiff materials resist deforming when you poke them. In contrast, some materials give way so dramatically to the slightest touch that they are said to have an exotic property known as negative stiffness. When pressed, such materials break apart or suddenly rearrange themselves internally, ending up as materials with ordinary, so-called positive stiffness.
Now, scientists have found that morsels of negative-stiffness materials can remain intact within larger chunks of positive-stiffness material. Moreover, the inserts give the host material a positively awesome boost in stiffness.
"If you combine negative stiffness with positive stiffness, you get very unusual behavior," says Roderic S. Lakes of the University of Wisconsin-Madison.