A blonde, a brunette, and a redhead are stranded on an island. The redhead looks to the east and says that the shore is about 20 miles away. She swims out 10 miles, but gets tired and drowns.

The brunette also looks to the east, figures the distance to shore is about 20 miles, and starts swimming. She gets 15 miles, but gets tired and drowns.

The blonde figures about 20 miles to the shore also. She swims 19 miles, to the point where she can barely see the shore, and gets tired. So she swims back.