Answer:
The molecules are trying to achieve equilibrium.
Explanation
In cells, molecules will <em><u>migrate through the phospholipid bilayer</u></em> during passive transport, a form of simple diffusion that does not use the energy currency, ATP.
This layer serves as a selectively-permeable membrane; its hydrophilic heads are drawn to water (seen looking outward), while its hydrophobic water-repellent tails face each other, allowing water molecules to diffuse <u>down the concentration gradient</u> through the membrane.
This typcally continues til the molecules gain equilibrium or balance. At equilibrium, molecules neither enter nor leave the cell through passive transport.