This is my theory. Lets say they are perfectly fine and they eat grass. Nothing will probably happen. If they have an upset stomache though, then they will probably throw up. When Oscar got sick (I think he ate cardboard =/ silly dog) we came home and there was puke all over my aunt's hardwood floor. It had cardboard and grass in it so we figure he ate the grass to help him. They said he doesn't usually eat grass. So maybe some dogs just like grass and some don't.