Oil!

The young philosopher decided that he had had enough, and started to swim in; but then he noticed someone on horseback, galloping down the beach towards him. The figure was bareheaded and clad in knickerbockers, and appeared to be a man; but you never could be sure these days, so he swam and waited, and presently made out that it was Vee Tracy. She saw him, and waved her hand, and when she was opposite, reined up her horse. "Good morning, Mr. Ross."

"Good morning," he called. "Is this part of the doctor's prescription?"