Answer
A lipid is amphipathic, and thus it will automatically form a membrane when in water.
Work Step by Step
Lipids are amphipathic with a hydrophilic phosphate head and two hydrophobic fatty acid tails. Thus, when placed in water, a bilayer will form because the hydrophilic heads will stick out since it is attracted to water, and the hydrophobic tails will stick in because it is nonpolar.
