DNA
DNA is a long fiber, like hair, only thinner and longer. It is made from two strands that stick together with a slight twist something like this:
Everyone's DNA is pretty similar. Everyone who has blue eyes has almost the same code for blue eyes. Everyone who has brown hair has almost the same code for brown hair. This coding sequences are separated by 'junk' DNA. This 'junk' DNA is non-coding and only serves to separate the coding sequences. These 'junk' DNA sequences are random and unique to an individual. DNA is used to help identify people of doing a crime.