Shiv LINGA - the linga in this word doesnt resemble human organ.
Linga is the resembleance of Shiva which as per hindu mythology is the power what every human have it his head. There is a big current around this of power within our nervous system. ( one of the reasons why we touch our head during sashtang Namaskar or Niskar is to flow this current back to earth)
To experience it you need to awake the nadis. A subject which is very vast and can be acheived through meditation.
This is why the Linga is been used to worship.
To overcome difficulties in igniting the fire and make their life easy while worshipping, people started making use of an image of flame (called 'ling' or 'linga') in place of the actual live fire. This was carried out by choosing a linga drawn or sculptured roughly in the shape of a live flame symbolizing the fire in a yajna.
