A married couple have been stranded on a deserted island for many years. One day another man washes up on shore. He and the wife become attracted to each other right away, but realize they must be creative if they are to engage in any hanky-panky. The husband, however, is very glad to see the second man there. "Now we will be able to have three people doing eight hour shifts in the watchtower, rather than two people doing 12-hour shifts."
The newcomer is only too happy to help and in fact volunteers to do the first shift. He climbs up the tower to stand watch. Soon the couple on the ground are placing stones in a circle to make a fire to cook supper. The second man yells down, "Hey, no screwing!" They yell back, "We're not screwing!"
A few minutes later they start to put driftwood into the stone circle. Again the second man yells down, "Hey, no screwing!" Again they yell back, "We're not screwing!" Later they are putting palm leaves on the roof of their shack to patch leaks. Once again the second man yells down, "Hey, I said no screwing!" They yell back, "We're not screwing!"
Eventually the shift is over and the second man climbs down from the tower to be replaced by the husband. He's not even halfway up before the wife and her new friend are hard at it. The husband looks out from the tower and says, "Son-of-a-gun. From up here it DOES look like they're screwing.