A phospholipid is comprised of a phosphate hydrophilic head, which means that it is "water-loving," and a fatty acid hydrophobic tail, which is "water-hating." The head and the tail are joined together by a glycerol molecule. The phosphate head is attracted to water because it is charged (i.e. negatively). Water is a polar molecule, which means that there is an uneven distribution of charges within its molecular structure with the oxygen side being "more negative" than the rest of the atom (which is "more positive" near the hydrogen). Thus, the negatively-charged nature of the phosphate head and the parts of the water molecule which are positively charged enable the two to form an "attraction" towards one another.
On the other hand, the hydrophobic tail is nonpolar, which means that it does not have a "more positive" or "more negative" side or part in its molecular structure. These differences in structure with water make the hydrophobic tail unattracted to water molecules and more attracted to other uncharged, nonpolar molecules (such as fats and oils).
An independent variable is the only variable that is changed while the rest is kept the same. In this setup, the doctor is using different creams on the same patient with the same skin condition so the independent variable is the skin cream and not the skin condition.
