The Perpetual Cassette Recording (PCR)
Let's consider the following scenario. A master recording by (insert name of favorite recording artist) is produced in a sound studio. You purchased one of millions of cassettes that was an exact duplicate, song for song, of the original. Let's say you want to find a specific song on this audio tape and you wish to tape it over over and over again so you have many copies of your "targeted song". You place it into the left side of your dual cassette player and press the "seek" button which will find the section immediately preceding the targeted song. This is analogous to the primer which locates the beginning of your targeted sequence in PCR methodology. Now take a blank tape and place it into the right (dubbing) side of your dual cassette player. Pressing "record" allows the the music from your targeted song to be recorded, and the magnetic coding sequences that are being copied onto new tape is comparable to the elongation process in PCR according to the original DNA template for that particular sequence. The "stop" button is analogous to the second primer which signals the end of the targeted sequence.
Even though this is not an exact analogy of the PCR process which will allow exponential replication of a DNA sequence, the purpose of this analogy is to show how a linear ordered sequence can be targeted and duplicated over and over in an ordered manner. Remember that the magnetic coding on the audio tape is just coded information that gets copied over and over again. In order for this coded information to be expressed the tape has to be placed in a cassette player and translated into music just like the cell's coded DNA sequence has to be ultimately translated into protein in a cell environment.