Sometimes when I have my morning coffee, which I drink black with no sugar, I notice a thin film floating on top of the coffee.
I now realize the nature of this substance, and so I run to my wife screaming, "Look at this in my coffee!"

Hydrophobic substance

Explanation:

The coffee is a brown color powder that has a bitter taste. It is a member of the purine derivative methylated xanthines. It is a principle source of caffeine. After digestion, of caffeine in the body it get rapidly absorbed in the blood stream.

The coffee is a moderately soluble substance thus some components remain hydrophobic when dissolve in water or milk. These components form a layer over the surface of the fluid.

Based on the above description, a hydrophobic substance is the substance which can be seen as a film over the black coffee.

b.have there own function.