Oskar tells us that his performances under grandstands weren't his only way of manipulating people during his childhood. He also liked to use his voice to cut holes in the windows of shops at night.
But Oskar doesn't cut these holes so he can steal stuff himself. He cuts them because he wants to tempt passersby to steal things.
Let's say you're walking down the street one night. There's nobody around, and you suddenly stop to gaze at that awesome watch that's sitting in a shop window. Then clink, a tiny part of the window—just big enough for your hand to fit through—falls out.
Of course, some people steal stuff and some don't. Oskar's always disappointed with the ones who are too honest to steal even when they can get away with it. He sees himself as kind of like the snake who tells Eve to bite the apple in the book of Genesis.
One night, Oskar sees his Uncle Jan walking past a jewelry store window. Oskar knows that Jan would never steal anything for himself.
Oskar gives Jan the opportunity to steal a necklace that he can give to Oskar's mom. That's something he'll definitely steal, and he does.
After he's stolen the necklace, though, Jan looks around and sees Oskar standing across the street. He knows the whole thing is Oskar's doing.
But he just crosses the street and walks home with Oskar, not saying a word about the necklace in his pocket. He gives it to Agnes.
Oskar tells us that after the war, he traded the necklace for cigarettes and a briefcase.