What's happening to the envelopes??

Sometimes my mom uses this machine that looks like it's eating envelopes and pooping them out of the top. I find it really fascinating, and I can't seem to figure out how it can eat them and then poop them out so fast! When I eat something, it takes at least a day for it to come out, and it's usually stinky! Mom won't let me get my paws in there, so I have to be content with just watching. Does anyone know the answer to this mystery?? And why would anything want to eat envelopes?