Recognize and name triangles and rectangles, and tell a rectangle apart from a square.
A triangle has 3 straight sides and 3 corners.
A rectangle has 4 sides and 4 corners too, like a square, but two sides are longer than the other two.
Mastery looks like: Child correctly names triangles and rectangles and can tell a rectangle apart from a square by its side lengths.
Common stumbles: Child calls every four-sided shape a square, without checking whether all four sides are equal.