"All squares are rectangles" — knowing this, Prisha thinks it can also be flipped around to say…

Why

Prisha is treating 'all A are B' as something that works the same in both directions — a reasonable guess for many everyday sentences, but not one that holds for group membership. 'All squares are rectangles' says every square belongs to the bigger group of rectangles (a square is, after all, just a rectangle with all four sides equal) — but that only means the SQUARE group sits entirely inside the RECTANGLE group, not that the two groups are the same size. Test it: picture a rectangle that is 2 cm by 4 cm — it has four right angles and opposite sides equal, so it is completely, genuinely a rectangle, yet its sides are not all equal, so it is not a square at all — a real rectangle that breaks the flipped statement. Her brother is right: 'all squares are rectangles' is true, but flipping it to 'all rectangles are squares' is false — 'all A are B' places A inside B, it does not make A and B the same group.