Once Buddha was walking from one town to another town with a few of his followers. This was in the initial days. While they were travelling, they happened to pass a lake. They stopped there and Buddha told one of his disciples, “I am thirsty. Do get me some water from that lake there.”
The disciple walked up to the lake. When he reached it, he noticed that some people were washing clothes in the water and, right at that moment, a bullock cart started crossing through the lake. As a result, the water became very muddy, very turbid. The disciple thought, “How can I give this muddy water to Buddha to drink!” So he came back and told Buddha, “The water in there is very muddy. I don’t think it is fit to drink.”
After about half an hour, again Buddha asked the same disciple to go back to the lake and get him some water to drink. The disciple obediently went back to the lake. This time he found that the lake had absolutely clear water in it. The mud had settled down and the water above it looked fit to be had. So he collected some water in a pot and brought it to Buddha.
Buddha looked at the water, and then he looked up at the disciple and said, “See what you did to make the water clean. You let it be ... and the mud settled down on its own – and you got clear water... Your mind is also like that. When it is disturbed, just let it be. Give it a little time. It will settle down on its own. You don’t have to put in any effort to calm it down. It will happen. It is effortless.”
“It is effortless.” Having 'peace of mind' is not a strenuous job; it is an effortless process.