-> What keeps the can from collapsing is the tapered form at the top and 
-> bottom. Also, as you say, the pressure inside keeps the walls under 
-> tension, until the pressure is relieved. But it's the fact that liquid 
-> can't be compressed that makes the can rigid. Once there is an exit place 
-> for liquid to flow, it will no longer stop the can from collapsing. 
  
That would be true if the can were absolutely full of liquid. But I'm 
sure that, in most cases at least, there is some gas at the top. That's 
why an unopened can makes a sloshing sound if shaken. The gas is 
compressible, so the can can be compressed at least slightly, if enough 
pressure is applied. But, since the gas is at quite high pressure, this 
is not easy. 
  
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