The washer bottle (or reservoir) is a translucent plastic tank typically mounted in the front fender or engine bay. It holds the windshield washer fluid (water mixed with detergent and antifreeze). It is molded to fit into tight spaces around the wheel well. Over time, the plastic can become brittle from heat and age, leading to cracks that cause the fluid to leak out as soon as you fill it. It has mounting points for the washer pumps and a filler neck with a cap to keep contaminants out of the fluid lines.