Who should be responsible for it?

One day Mark lost his favorite train. He asked me to help him find it out but I refused. Then he cried.

"Who played with the train? Who lost it? Who should be responsible for it?" I asked.

Mark wept and answered "me" three times in a quiet voice.

"Listen to me," I saw him still weeping and said, "Mark, I bought you that train and hoped you could cherish it, but you lost it. It's me that should be angry instead of you. If you cannot take care of it, nobody can help you. Remember you have responsibility for all your toys. I know you are quite sad. I can wait for you to recover, okay?"

He nodded his head and ceased crying gradually.