In Geometry Square, there are two houses right next to each other, covered in lightbulbs (2 large, 5 small). The goal is to get all the lights lit.
Knock on one of the doors (it doesn't matter which one you start with), and make note of what the occupant says (subtitles are handy here), and what light(s) come on. If a big light comes on, and the result is positive (ja!, jaeh!), knock on the door where the big light came on (even if it's the same door you just knocked on). If the result is negative (nah-ah), knock on the door without a big light (even if it's the same door you just knocked on). If a small light turns on, knock on the opposite door from the one you just knocked on.
Once all lights are lit, you'll get the memory and the achievement.
I had a lot of trouble with this one, and all the guides I found didn't do a very good job of explaining it. I tried to make it as simple as I could.