You are home alone, and you hear on the news about the profile of a murderer who is on the loose. You look out the sliding glass doors to your backyard, and you notice a man standing out in the snow.
He fits the profile of the murderer exactly, and he is smiling at you.
You gulp, picking up the phone and holding it to your right ear and shaking, dial 911. You look back out at the glass as you press the phone to your ear, and notice the man is much closer to you now.
You drop the phone in shock. There are no footprints in the snow.
There is nobody in the garden.
It's just your reflection.