The cell membrane is a phospholipid bilayer, which means it has two layers of lipid consisting a phosphate group. Both layer contain water loving (hydrophilic) exterior or "heads" , and water hating (hydrophobic) interior or "tails". The watering loving heads are at the exterior because the outside environment usually contains water. That is why in the picture all water hating tails avoided themselves from the water gun. By the way the pear shape huge object in the picture is called intergral protein, it's embedded within the membrane and serve as transporters, carriers or receptors. This phospholipid bilayer property of the cell membrane often causes big polar (always water loving) molecules not able to pass through the water hating lipids in the membrane, so some specific proteins serve as channels or carriers. Joke (pic) credits to: https://i.pinimg.com/1200x/5a/46/00/5a4600d58802d846bf8f6fe4edc79915.jpg
