You will need squared paper for this activity.
Start by drawing a set of axes like this:
Then, draw the following lines:
From (1,1) to (4,5)
From (4,5) to (8,8)
From (1,1) to (5,4)
From (5,4) to (8,8)
Then measure each of the four lines accurately.
What shape have you drawn?
Now, measure the distance from the bottom left to the top right of your picture (the long diagonal of the shape).
What is the length of the short diagonal?