Why is the phospholipid bi layer only partially permeable to water?

The phospholipid bilayer is only partially permeable to water because its hydrophobic core, formed by the fatty acid tails of phospholipids, repels polar molecules, including water. While water is a small molecule and can pass through the bilayer to some extent, its movement is limited due to the hydrophobic environment. Additionally, specific protein channels, such as aquaporins, facilitate the transport of water, allowing for regulated permeability rather than unrestricted passage.