Sims need plumbing. Some commentators on the game regard it as "all about potty training" or, as a recent edition of PC Format magazine put it, "advanced Bladder Control".
Be that as it may, plumbing is probably the first furniture you will buy for your Sims in a new house, and very likely the first item they will use!
Follow the links below; click on the pictures to download, extract the IFF file into \Program Files\Maxis\Downloads\