A childhood belief about weeing and pooing

Presumably this happens to everyone, (unless you're in an outhouse, or a l-o-n-g way above the water).
Payment for the relief you get is often taken immediately in the form of a cold splash, usually dead center, of whatever liquid is in the bowl, I always hope it is just clear water, not mixed with anything yellow.
Which brings me to what I used to believe.
I used to believe that, when doing 'the big job', our body was programmed to do it's business first, and only then did our bladder get it's turn. It worked that way most of the time. I marvelled at the 'rightness' of such an arrangement.
I must admit, though, I still wonder...