Your bladder is a stretchy, balloon-like organ inside your body that stores urine until you're ready to use the bathroom. When you drink water or juice, your kidneys filter your blood and send the waste liquid down tubes called ureters to your bladder. The bladder expands as it fills, like a water balloon getting bigger. When it gets full enough, nerves send signals to your brain that it's time to go, and the muscles in your bladder squeeze to push the urine out.

A healthy bladder can hold about two cups of liquid for several hours. Babies can't control their bladders yet, which is why they need diapers, but as kids grow, they learn to recognize the signals and hold it until they reach a toilet. If you've ever desperately needed a bathroom during a long car ride, you've felt your bladder working hard to hold on.

The word bladder can also refer to any similar stretchy sac that holds liquid or air, like the inflatable bladder inside a soccer ball or basketball that gives it bounce.