3D glasses

As I was carried away with the movie, sometimes wowed by the breath-taking reality of 3D pictures, suddenly I wanted to know how this wonderful special effect was working. I took off the 3D glasses. The picture blurred just as I thought. That's reasonable because both eyes were seeing both pictures for right and left eyes. Ok, then, I turned the glasses upside down so that my right eye looks through left lens and left eye right. Oh, it's great. The things that should be farther from me looked closer and the closer things farther. hahaha. It's fun indeed but only drawback was... it spoiled the movie. Then a question occurred to me. What switches the shutters for the left lens and right lens on and off alternately ? It must be synchronized with the pictures for the corresponding eye showing by turns. I guessed that it was done wirelessly because there was no wire connected to, say, armrests. Ok, now I know all the technological aspect of the 3D movie.
When the movie ended, my wife, my son and I walked out the theater talking excitedly about the 3D effect. I told them my experiment with the glasses. Just as I proudly explained the synchronization mechanism supposedly done wirelessly, my son said
"Dad, your wrong. It's done perhaps optically through a small window at the middle of the glasses between the two lenses. When I put my finger over it, 3D became 2D and the picture got blurred."
Oh, he also conducted some experiments. ... Like father, like son.