http://www.physicsclassroom.com/Class/sound/u11l2c.cfm according to this,rubber is not elastic The problem is that I read before that elasticity is the physical property of the material when it deforms under stress(eg external forces),but it returns to its shape when the stress is removed. An object is said to be elastic if it restores its original size and shape. but rubber is elastic because it restores its original size and shape.. I'm confused I fell that this is contradicory Is elasticity constant for all solids? its written in the text book that "the speed of sound increases in solids and liquids by increasing their densities due to the increase in their elasticities" is that true? If so ,Is there a relation between elasiticity and density?