Measure 4 inches across the length of the foam block from one end and make a marking at the top edge with a pencil. Hold a protractor against the marking and make a mark at the 45-degree angle. Draw a straight line through the 45-degree angle from the 4-inch marking down the width of the block using a straight edge.
Measure 16 inches across the length of the foam block from the same end and make a marking at the top edge with a pencil. Hold a protractor against the marking and make a mark at the 135-degree angle. Draw a straight line through the 135-degree angle from the 16-inch marking down the width of the block using a straight edge.
Cut the foam block along the marked lines with a sharp knife. Draw a line around the cut blocks roughly 1 inch from the top using a black marker. Draw a second line around the cut block roughly 1 1/2 inches from the top using a black marker. Fill in the section in between the lines with your choice of colored marker to create a reference line.
Push the blocks together to raise the center block to demonstrate plate compression. Hold the blocks together above a table and slowly pull them apart to sink the center block, demonstrating plate drift.