To fix the error, start by creating a new function called updateElement. You may recall that we added this to the body's onscroll method. Now that we have distributed our objects in a random fashion on the screen and created a smooth scroll effect, we need to work on making things change at different rates as we scroll through the rows. This is the meat and potatoes of the parallax effect. In the end, when you scroll through the page, the objects can be programmed to move at different velocities.

This part will involve some fancy footwork and will be explained in a few steps, but it will be lightweight and easy to extend later as you please. The following code contains a function that will be run on every scroll, so we want to keep it as light as possible.

Inside the new updateElement function, in the first line, we will create a call to another function, sending an anonymous callback function. This function will get the elements we need to move and send the...