Salt water
A traveling salesman had to walk so much that his feet often hurt. His doctor told him that salt water was the best thing for them, so the salesman decided to go to the sea for his vacation that year. Since all of the hotels near the sea were expensive, he went to a small hotel far away from the beach.
In the morning he went down to the calm sea with a bucket（水桶）, went over to the lifeguard and asked whether he would be allowed to take a bucket of salt water. The lifeguard seemed very surprised but said, "Yes, although you'll have to pay twenty-five cents for it."
The salesman gave the lifeguard twenty-five cents, filled his bucket, took it to his hotel and put his feet in the water.
After lunch, he came down to the beach again. The tide had gone out now, so the sea was much lower. The salesman thought, "That man has a very good business. He must have sold thousands of buckets since this morning."