water can be a cure too, and as most people ahve ready access to water at 8:30 on a sat night, it seems the best to me:)
There is a constipation cure (medicine) called movicol, which works on the exact principle of adding water to the stools - its on prescription and you mox it with a glass of water. It works not by doing anything else but making the water not absorbed by the body/peed out, but by making sure it's added to the poo. Interestingly, i was once told it was discovered as a chemocal when they used it on the mary rose, to make sure it didn't dry out!