The real challenge is a human face formed by the trees, branches, shadows, and patches of snow. It is not drawn separately, so you need to look at the entire scene rather than concentrating on individual objects.
This illusion works because the brain quickly identifies familiar patterns. Once it decides it is looking at a forest and a deer, it becomes harder to notice that the same shapes can also form a completely different image.
After you finally spot the face, it suddenly seems obvious. The puzzle shows how strongly our first interpretation can control what we notice, even when the hidden image has been in front of us the entire time.
