Say you had not go for several days, your muscles would have compacted it, pressed out all of the gas and then, produced a dense brick of poo. But on the otherhand, say you were very regular, and had eaten foods that produce large amoutns of gas, this would be trapped in the poo. And it would cause it to float.

The natural law of defusion states that a gas of a higher concentration will always migrate to an area of lesser concentration. Which is why you fart. But if you poo before you have reached a state of equilibrium of internal pressure, then you are facilitating that law.

In the future, try not to worry so much about the crap you cannot change. Lol.

That is my theory and Im stickin to it!!

I did some research and I found this page:

http://findarticles.com/p/articles/mi_m0NAH/is_3_29/ai_54189553

I hope that answers your question.