"It has to do with the design of the bladder," Hu tells Popular Science. In other words, it's all in the urethra, the bodily pipe that moves urine from the bladder out of the body. (In dudes, it also ferries sperm.) "It's really important for making the bigger animals eject fluid faster," he says. "It's the length of the pipe that matters." Because larger animals seem to use gravity, rather than the muscles of the bladder, to keep their flow going, the longer the tube, the faster a single molecule moves through it. The diameter of the urethra matters, too, by increasing the volume of liquid that comes out every second—in the elephant's case, that 1.5 gallons.