Right! It's the square. The square with its four equilateral sides
and angles has the biggest area of all possible
rectantgles of a given perimeter.

Now let's look at a different shape. For example the triangle. Since
it's not quite as easy to calculate the area for a triangle, you can
go right to the magic picture. Move your mouse around the picture
below to make different triangles. Here again, the sides always add
up to the same length, this time 24 units. What is the relationship of
the sides and angles in the triangle with the biggest possible area?