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It's one of the many mysteries of toilet time, why do some poos float, while others sink?

Well, now scientists have taken a fresh look at a 50-year-old study into poop that looked at fat content and gas inside human droppings - and have come up with what could be an answer.

The research found, and this might put you off your sandwiches, that when poos that float are smushed-up, they sink to the bottom.

And that, according to experts, is because floating poos have a high gas content, so floating or sinking "depends upon differences in gas rather than fat content".

And that has got everything to do with gut bacteria.

But why does some poo have more gas inside than others?

Another good, but yukky, question.

Researchers have been examining why some poo has more gas in it during a study of mice.

Around half of mouse poo floats, but the scientists discovered that poo from mice that lack gut bacteria - known as germ-free mice - tended to sink.

To find out more, the team took gut bacteria from healthy mice and put them into the stomachs of germ-free mice.

The results saw an increase in floaters, showing that whether a poo floats or sinks is all to do with bacteria in the gut.

"Now, there's no confusion as to what makes stool float, it is gas from gut microbes, not from swallowed air or other sources," said Nagarajan Kannan, one of the researchers from Mayo Clinic in Minnesota, USA.

However diet, genetics, or even how a person was born may still play a role in determining poo-types, says Kannan.

That's because all of these factors are known to influence the amount of bacteria found in a person's gut.