If you manage to crack your screen by jabbing it with your thumb, I would expect the ghost of Bruce Lee to appear and ask you to teach him your way of kung fu.  The PSI required would liquefy your thumb.<p>I could maybe see accidentally bending your phone with a death grip.  But, even that requires a serious strain: <a href="http:&#x2F;&#x2F;i.ytimg.com&#x2F;vi&#x2F;FwM4ypi3at0&#x2F;maxresdefault.jpg" rel="nofollow">http:&#x2F;&#x2F;i.ytimg.com&#x2F;vi&#x2F;FwM4ypi3at0&#x2F;maxresdefault.jpg</a>