Proverbially, you can't....

... have it both ways
...have your cake and eat it
...get blood out of a stone
...make an omelette without breaking eggs
...make a silk purse out of a sow's ear
...run with the hare and hunt with the hounds
...teach an old dog new tricks
...judge a book by its cover
...shake hands with a clenched fist
...tell which way the train went, by looking at the tracks
...win arguements by interrupting speakers
...have a rainbow without rain
...pick up two melons with one hand
...fool all of the people all of the time
...sip soup with a knife
...see the sky through a bamboo tube
...measure the sea with a shell
...cheat an honest man
...catch a cub without going into the tiger's den

-schott's original miscellany, one of my fav books...