Knots occur when roughened cuticles on separate hairs open up, overlap and interlock. This is usually as a result of damage caused by chemical processing, excessive use of heated appliances and blowdrying. Rough treatment with brushes, tools and even drying with towels can also cause breakage and upbraiding of the cuticle that should normally lie closed and smooth.
To eliminate knots without breaking even more hair, apply lots of conditioner and work through individual sections with a wide-toothed, smooth plastic comb. Start from the ends up, holding your hand against hair around the roots to prevent pulling it out. Then rinse conditioner out with a gentle flow of water. Be sure to avoid too powerful a force which could tangle up hair all over again.