Proverbially, you can't...

...have it both ways
...have your cake and eat it'
...get blood out of a stone
...make an omlette 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 track
...win arguments 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 tigers den