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)