Optical Illusion

This is a pretty famous optical illusion. It works very well and it's so easy that your kids can draw it herself/himself, what makes it even more stunning.

Step 1: Tell your kids to draw lines next to each other, both exactly equally long, like this:

Let her/him check that lines are really equally long.

Step 2: Add arrowheads and "backward" arrowheads to the lines, like in the following image. Depending on the age of your kids let her/him do it or draw it yourself. 

Now examine the initially drawn lines. They are still equally long. REALLY? Well, these arrows play tricks on your eyes...