Ya?l: Well, let's see if we can figure this mystery out. The first clue is that the density of the human body is similar to the density of water, and what keeps us floating--other than the dog paddle--is the air in our lungs.
D: Okay, so you'd think a corpse would sink as the air in its lungs is replaced by water.
Y: Well, they do--at least initially. But when any organism dies, it goes through putrefication, the series of chemical, physical, and biological changes that end up returning the body to the food chain. That's your second clue.
D: Hmm! I still don't get it.
Y: Okay, so here's your third clue: when a body dies, the bacteria that normally live in the digestive system continue to feed on the proteins and sugar in the body's soft tissues, and to excrete gasses including carbon dioxide, ammonia, hydrogen, and methane.
D: I give up. What does that have to do with floating?
Y: Well, these gasses accumulate in the body's cavities, and--if it's submerged in water--eventually cause it to start rising.
D: Gross. So why do the villains always drown their victims?
Y: Well, decomposition happens more slowly in water, and even more slowly in cold water.
D: So the mystery's solved.
Y: Took you long enough!
