Scientists explain mirror reversal as a geometric reflection across a plane. The key is that a mirror swaps the direction perpendicular to its surface (front-back), but leaves the other directions (left-right and up-down) unchanged. Our brain interprets the mirror image by imagining it as another person facing us, which creates the illusion of left-right reversal. This understanding is backed by physics and optics, and helps explain why text appears reversed in a mirror but not upside down.