When I was a child, I had a Sonic the Hedgehog cake for one of my birthdays (don't remember which one). Most of my immediate family had some of it, and all of us were pleased to announce we had blue poo the following day.
Of all the hospital tests I've had to find out what's wrong with my gut (I have finally been diagnosed as having Irritable Bowel Syndrome, which seems a bit of a cop out to me), one of them was a barium meal. A barium meal involves swallowing a very dense "liquid", that can be seen on X-rays whilst passing through you. The following day I had what could only be described as a concrete turd. It stuck to the bottom of the toilet for about a week afterwards.