The head of a phospholipid can be descibed as

Phospholipids are a class of lipids and are a major component of all cell membranes as they can form lipid bilayers. Most phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such as choline. 

Amphipathic character 
The 'head' of a phospholipid is hydrophilic (attracted to water - polar), while the hydrophobic (non-polar) 'tails' repel water. The hydrophillic head contains the negatively charged phosphate group, and may contain other polar groups. The hydrophobic tail usually consists of long fatty acid non-polar hydrocarbon chains.

They both determine the exact age of rocks.