What part of the cell is made mostly of waterproof liquid material?

The part of the cell made mostly of waterproof liquid material is the cell membrane, also known as the plasma membrane. It is primarily composed of a phospholipid bilayer, which creates a hydrophobic barrier that separates the internal environment of the cell from the external surroundings. This structure helps maintain the cell's integrity and regulates the movement of substances in and out of the cell.