Kidney stones are so small that once they get from the kidney to the bladder, they usually pass the rest of the way out unoticed. Bladder stones may be so big that they can't get out, except through a surgical incision. Some kinds of stones can be dissolved with the right diet, but not Theo's. Some of them are small enough to pass when the dog urinates. It's the ones that are almost small enough to pass that cause pain.
Male dogs have a bone in the end of the penis, with a groove in it.
To get out, the stone has to fit through the groove, and it often doesn't. Male dogs are at risk to become completely plugged up by a bladder stone stuck in that little groove. Female dogs get stones, but rarely get plugged up. The world just isn't fair. But Theo came in today because of a skin problem.