So when the poo leaves the plane, where does it go? Sometimes, it hits people on the ground. But thankfully, not always.
A video by Brit Lab explains everything you've ever wanted to know about airplane toilets -- and probably some stuff you didn't want to know. Basically, pressure in that vacuum toilet moves your waste down to a holding tank at the back of the plane. Normally, this poo does not go whooshing out into the air, unless the tank has a leak. In that case... bombs away.
That's the short answer, but hear it all explained in greater detail in the video above. The only thing it doesn't explain is why people go to use the toilet in just their socks. That's gross.