You see, the thing is that we’ve got these things called bladders that store the waste fluids our body has so far filtered out so that we’re not constantly dribbling a bit of piss and making it trivial for predators to track us. When that bladder gets full and stretches, it is detected by some nerves and then our brains signal that it’s time to find a toilet or fridge to piss in.

Hopefully that relieves the pressure of your curiosity a bit.