už

In the first week of class I learned the expression:

Nėra už ką [nera uš kaa]

You say it to be polite after someone thanks you, where in English you'd say you're welcome or pole tänu väärt in Estonian.

nėra = is not (= ne 'no(t)'+ yra 'is')
už = for
ką = ACCUSATIVE of kas 'who/ what'

It translates something like: 'there is nothing to be thanked for'. When ką has the meaning 'for' it causes the following word to take the ACC case but ką can also mean 'behind' and when this is the case it causes the following word to take the GENITIVE.

Saulė yra už debesų.
The sun is behind the clouds.
Päike on pilvede taga.

debesų [GEN pl.] <--- debesis [NOM sg.] 'cloud'
ką [ACC] <--- kas [NOM] 'who/ what'