"Sa du sten?" is a question/funny sentence in Swedish. It's a popular "internet quote" in Sweden. The origin of this quote is from a swedish parody clip (watch?v=MPlIMrAZ058) of the movie "Alla vi barn i Bullerbyn."
"Sa du sten?" is mainly used just to be annoying.
"Hey, can we go to the bakery and buy some bread?"
"Sa du sten?"