"Make a Square" activities extend and review "Make a Rectangle". In the books, the "Make a Square" activities include some cases which do not yield a square, plus some puzzles where the constant term is missing, and some where the linear (middle) term is missing. This helps students see the structure of perfect square binomials. After doing those activities, you can use the Squaring a Binomial applet to discuss that structure and solidify that understanding. And you can follow up with a writing assignment based on the Completing the Square applet. I wrote a teacher's guide to this topic as a mini-unit: Completing the Square.