system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it when I wear 3d glasses on top of my glasses, it works. But, when I wear my glasses on top of the 3d glasses, it doesn't?

Glasses is spelled weird.
gpt
3d glasses work using polarized light.
The projector shoots light a the screen with two pictures, one using lines that go up and down and the other with lines that go from side to side. The 3d glasses sort the lines, so that your left eye sees only lines that are side to side and your right eye sees only lines that go up and down.  That means each eye gets a slightly different picture, the way they would if you were looking at an actual 3d object. That's what fools your brain into thinking the flat screen is 3d.  
Your glasses then bend the light so that it hits your eye with correct focus. To do that well, they have to be the correct distance from your eye.  
Putting your normal glasses outside the 3d glasses likely messes with the distance. Some glasses also have coatings that affect the polarization (the up/down and side-to-side) of light. 
So you could be looking at 1) a picture that is improperly focused and fails to fool your brain, or 2) a picture where the polarizing effects of your normal glasses ruins the sorting function of the 3d glasses.