Size and Space Variation
Here are some fence posts marching across the landscape. The top row of posts does nothing to add depth to the picture.
But suppose we change the posts' relative sizes so that we have something like the bottom row. It's apparent that the leftmost post is closest to the viewer and the ones to the right are farther and farther away. This is the psychology of how we see things: if you know certain objects are normally the same size, but you see a picture in which those objects are not the same size, you'll automatically and unconsciously assume that some of those objects are farther away than others.
There's another difference ...