There is no way that the 2 thick, black lines are the same height, right? Let's look a little further into it and see.
Here's a video that showcases and explains the full effect of the illusion:
Step 1: Materials
1) A piece of 8.5 x 11 paper(cut to 8.5" x 8.5")
4) Two crayons or colored pencils(preferably orange and yellow)
Step 2: Beginning the Drawing
-Draw a 3" vertical line 1" in from the left side of the paper, with the bottom of the line starting 2.75" up from the bottom of the paper. Make the line very bold.
-Draw an identical 3" vertical line 6" in from the left side of the paper, with the bottom of the line starting 2.75" up from the bottom of the paper. Make this line very bold as well.
As you can see from the photo, it looks almost impossible for these two aforementioned lines to be the same height!
Step 3: Finishing the Drawing
This picture is worth a thousand words, and is a drawing that you can look at and duplicate yourself.
-Color the drawing as shown with crayons or colored pencils. The coloring helps to "sell" the illusion more than just a black and white version.
-The "grid" effect shown on the walls provides for a sense and feeling of depth in the drawing. This also "sells" the illusion well!