oh, English...

Idioms have got be the most difficult part of a language to teach a non-speaker, and that's what today's tutoring consisted of. The theme was "Food Sayings", and it went something like this, given the choice of 5 food phrases or words:
"I know someone who is really crazy. They're _beans__pie__nuts_."

There really is no easy way to even make an educated guess, once you think about it! I mean, whoever decided a piece of cake was "easy", or even pie for that matter? The particular student I was working with, quite the character, decided that English itself was "full of beans" and "nuts".
I assured him there were idioms just as puzzling in his own language...

Norwegian: å gå som katta rundt den varme grauten
to pace around hot porridge like a cat = beat about the bush

Russian: Вешать лапшу на уши (Vešat' lapšu na ušy)
To hang noodles on one's ears = to tell lies / talk nonsense

Tibetan: chang.sa.rgyag
to put up a beer tent = to get married

French: pédaler dans la choucroute
to pedal in the sauerkraut = to spin your wheels - to go nowhere

And the best of all!
Dutch: Ik zweet peentjes
I sweat carrots = I'm sweating like a pig 

(from a fantastic language resource website, omniglot.com)