Anatomy of a Rope
Useful Knots for Everyone to Know (website)
Pedigree Cow Hitch video
Knot tying is extremely useful. It has a long history in the realms of sailing and survival. The best rope to practice with is a 10 mm or 1/2 inch cotton rope. You don't want parachute cord or plastic ropes because they are too slippery and thin for learning on. I found a good way to practice them was on the back of a chair, on an iced-tea pitcher on my lap. Of course outside on a tree branch too!
"Rope is measured in two ways: by diameter or circumference. Ropes up to one inch are generally measured by the diameter and ropes over one inch by their circumference. This must be clearly understood, since a rope one inch in diameter has a circumference of approximately 3 inches, while a rope one inch in circumference has a diameter of 5/16 of an inch." (from the scout's Rope guide)
You might have a meeting discussing any knots the boys already know, how knots come in handy and can even save your life, and learn about the different kinds of rope and cord, including their strengths and uses. Then the boys can cut a length of rope and seal the end by learning to whip the ends or melt them to keep them from fraying. Then they can learn a few knots and if there is an interest, even learn the decorative knots and plaits.