There are 3 girls stranded on an island 20 miles away from the nearest land, a blonde, a brunette, and a redhead.
The redhead says, "Forget this, I'm going to swim to land." She swims 5 miles then drowns.
The brunette thinking the redhead made it says, "Forget this, I'm going to swim too." She swims 10 miles and then drowns.
The blonde thinks that both of the other girls made it so she decides to swim. She swims 19 miles, decides she is tired, and swims back...