(Preparation: Cut out the bottom of a shoe box, resulting in
a cardboard box that can bend at the corners.)
I introduce the square, rectangle, parallelogram, rhombus,
trapezoid at the chalkboard, noting the properties of the
sides and the angles.
To summarize the lesson, I hold the shoebox in front of the
class and say: "If you bend a rectangle like Gumby, what
quadrilateral do you get?" (parallelogram). Bending the
shoe box demonstrates the change in angles, and the fact
the length of the sides have not changed. I then ask: "If
you bend a square like Gumby, what quadrilateral do you get?"
The whole thing sounds silly -and that is exactly why my
students remember it so well!