A chemist, an engineer, and a physicist were stuck on a desert island with with one can of beans and no way to open it, so they all fell to thinking of ways to get at the food.
Suddenly, the chemist yells, "I have it! We can make a compound from the sea water and the sand that will dissolve the can and leave the beans untouched!"
The engineer replies, "No, no - I found some driftwood and seaweed, and I have designed a contraption that will lift the lid off the can!"
The physicist remains silent. After a while, the other two walk over to him and say, "Well? Don't you have a plan to get at the food?" The physicist replies, "First, assume we have a can opener . . ."