Have you ever noticed a pipe shaped like this under your kitchen sink? It is called a "trap" and it's job is to hold water. We want to keep water in the belly of that pipe because it seals out the smelly sewer gasses that would otherwise stink up your house. Your toilet has a trap also but it is built in to the toilet.

If you ever smell foul odors in a bathroom that hasn't been used in a while, just add a little water to the trap.