The plasma membrane is made out of phospholipids,and each phospholipid molecule has a head and two tails. The head "loves" water (hydrophilic) and the tails "hate" water (hydrophobic). The water-hating tails are on the interior of the membrane as they cannot be in contact with water, whereas the water-loving heads point outwards, toward either the cytoplasm or the fluid that surrounds the cell as they are in contact with a water-based environment. For the membrane to form in this way, it must be a bilayer.
Step-by-step explanation: