Thanks for the advice

Sometimes it takes a foreigner to speak English properly.

We say

Know your limitations

or

You can't get blood out of a stone

Polish people say

You can't jump higher than your cock

I'd like to point out that I know only one Polish person and he seems quite mad.

I used to know another one, a girl, but she didn't like me any more after finding out that I was 'boastful'.  

Normally it's because I'm a bastard or something, but whatever.