Things you can't do

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 sows 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
Have a rainbow without rain
Fool all the people all of the time
Pick up two melons with one hand
Cheat an honest man