These cells are continuously being sloughed off by your cheeks. Spit the water into your cup. Cap tube and gently rock it on its side for 2-3 minutes. The detergent will break open the cell membrane to release the DNA into the soap solution. DNA is a very long molecule.
Physical abuse can break it into smaller fragments, a process known as shearing.
Allow tube to stand for 1 minute. Place a thin acrylic or glass rod into the tube. Stir or twirl the rod in one direction to wind the DNA strands onto the rod. Be careful to minimize mixing of the ethanol and soapy layers. If too much shearing has occurred, the DNA fragments may be too short to wind up, and they may form clumps instead.