Slug Invasion

So this is going to sound ridiculous, but it will give you a glimpse of what us Watson's do when we are at home. I was walking through my kitchen late the other night and what do I see lurking across the floor, but a tiny little slug. Gross!! Quickly I scooped up the slug with a tissue and plopped the slug into the toilet thinking I was going to drown it. To my surprise the slug preceded to crawl up the toilet bowl. I called Joseph down and we watched as the slug slowly made it's way up and out of the water. It was at this point that we knew we had to take serious action. We grabbed the salt shaker and viciously frosted the slug. The slug fell back into the water as it foamed to a gruesome death. . . and we flushed the toilet. Problem solved. . . but here is my question: How did a slug get into my house?