A person was murdered on Sunday evening. A man was suspected, and the police interrogated him at Sunday night. He explained to the police officer that he had been at his favourite bar during the evening, and that he had been at that bar every evening during the last week, as usual. The police officer then challanged the man and said that if he got drunk so often is it not then possible that it would affect his memory and that he would thus not be able to trust his memory.
"No, there is no risk that I get too drunk too often" said the man. "You see, I am very regular there, it is like a ritual for me every evening. I go there, and then when I am finished, if I got too drunk, I move a stone one step to the right on the stone wall outside the bar, and if I did not get too drunk then I move that stone one step to the left. In this way I can keep a record and can have some kind of balance in my drinking habits. Every Monday evening, after my visit to the bar, I always start with the stone in its original middle position again, and move it one step to the right or to the left, depending on if I drank too much or not."
On Monday at noon the police officer went to the bar and asked some questions to the staff there. They said that they think the man was there yesterday evening, but that he is so regular that they hardly really think about him or notice him any longer.
When the police officer went out he saw the stone on the stone wall. It was two steps to the right. So he concluded that the man had been drinking a bit too much the last week.
At the police station they all discussed the case and the suspected man. And later on they arrested him.
Why was the man arrested?