Because of molecules we are connected to the outside world from our bodies. Like when you smell things, because when you smell a smell it's not really a smell, it's a part of the object that has come off of it, molecules. So when you smell something bad, it's like in a way you're eating it. This is why you should not really smell things, in the same way that you don't eat everything in the world around you because as a smell, it gets inside of you. So the next time you go into the bathroom after someone else has been there, remember what kinds of molecules you are in fact eating.