Brain Test Level 90 Answer
Tapping every switch over and over until the third lamp finally lights. One lamp never responds however often you tap it, so the level reads as broken rather than solved.
The puzzle
Please turn on all the lights.
The answer
Tap the switches to light the lamps that work, then drag the stubborn right lamp off the edge of the screen.
What most people try first
Tapping every switch over and over until the third lamp finally lights.
Why that is the answer
One lamp never responds however often you tap it, so the level reads as broken rather than solved. The wording is the whole puzzle. It asks for all the lights to be on, and a lamp dragged off the edge of the screen is no longer one of the lights in front of you. Remove the dead one and the two that work satisfy the sentence exactly as written.
If your level 90 looks different
Unico Studio shuffles level numbers between game versions and platforms, so the puzzle on your screen may carry a different number from the one here. If the question above is not the one you are looking at, the full levels 51 to 100 walkthrough lists every puzzle in this stretch by its question, which is easier to match than a number.