Stranded On An Island... Cannibals Involved?

There were three people stranded on an island, a brunette, a redhead, and a blonde. The brunette looked over the water to the mainland and estimated about 20 miles to shore. So she announced,

"I'm going to try to swim to shore." So she swam out five miles, and got really tired. She swam out ten miles from the island, and she was too tired to go on, so she drowned.

The second one, the redhead, said to herself, "I wonder if she made it. I guess it's better to try to get to the mainland than stay here and starve." So she attempts to swim out. The redhead had a lot more endurance than the brunette, as she swam out 10 miles before she even got tired. After 15 miles, she was too tired to go on, so she drowned.

So the blonde thought to herself, "I wonder if they made it I think I'd better try to make it, too." So she swam out 5 miles, ten miles, fifteen miles, and finally nineteen miles from the island. The shore was just in sight, but she said, "I'm too tired to go on."

So she swam back.