"What?" I couldn't help but swallow my saliva. 

"He always says that," my deskmate stared at me intently, as if he was the kid who saw a ghost. "Dad, there's someone on your back!" 

I was stunned. Suddenly, I had to pee, and my stomach was swollen. 

I stood up in a panic and rushed to the bathroom. My mind kept echoing with my sister's words that I had repeated for three years: "Sister, there's someone outside." - 5] 

That night, for the second time in my life, I didn't do my homework. I sat blankly at the desk, staring out the window. The night was not black, but a kind of lifeless gray, a kind of eerie blue. The leaves of the locust trees outside the window rustled and giggled, and the electric wires swayed uneasily from side to side. 

I felt as if countless tiny needles were drilling into my body through my pores. It was eerie and cold, very uncomfortable. I knew she was coming. Whenever she looked at me from behind, I would have this feeling. 

I turned around tremblingly, and sure enough, my sister was sucking her index finger and looking at me expressionlessly. Then, just like every time before, she took out her index finger, with a thin line of saliva, and pointed out the window: "Sister, there is someone outside." 

I turned around in horror and looked out the window, not missing any clues.

Yes, I'm twelve, older than six, so I can't see. I can't see what my silly sister sees. I thought, that gloomy, green-faced female ghost was floating outside the window, staring at me expressionlessly, just as I was staring at her. 

I suddenly realized that seeing a ghost isn't scary; what's scary is not being able to see it. 

You know exactly where it is, the danger is imminent, but you can't see it. If you can't see it, you can't hide, you can't defend yourself, you can't fight back. 

I stood up and backed away, step by step, toward the bed. 

My sister stubbornly pointed out the window with her arm. Then, stiffly and woodenly, she turned around, her arm still stretched out, pointing directly behind me. 

"That...that thing...is behind me?" I jumped up, slapping my back frantically and hysterically as if ants were crawling all over it. 

My sister smiled foolishly and slowly lowered her arm. Suddenly, she pointed at herself, then out the window. Then, she put her index finger back into her mouth, sucking on it with relish, and walked out the door. 

That night was a sleepless one for me. 

Why did my sister point at herself? Did she mean a ghost had possessed her? Or was the ghost herself? 

From then on, I began observing my sister from a different perspective. 

Whenever she was with my parents, she would become vacant, her eyes seemingly fixed on something, yet seemingly looking at nothing at all. 

But when she was alone, she would stare out the window with a silly grin, or stare blankly at the ceiling, or lie prone on the floor, searching under the bed and the sofa, as if searching for something important or interesting. 

That night, to prevent my sister from appearing behind me, I deliberately read facing the door, my peripheral vision constantly watching it. 

Suddenly, the window behind me made a slight noise. Startled, I turned around and stared out the window, where I seemed to see a small, dark shadow flash by. 

I cautiously opened the window and peeked out, but there was still only the locust tree outside. 

When I turned back again, I saw my sister, grinning foolishly. 

The light cast her shadow in a perfect, almost comical, shadow, a perfect combination of terrifying and utterly hilarious.