Sippy Cups

Why is it so hard to keep track of sippy cups? We even buy them in three packs , yet it seems like every few weeks they are added to our grocery list once again. How is it that I can keep track of laundry, all of the toys (although, we have definitely lost some of those as well), school readings and homework assignments, a child, and a countless list of other things, yet fail to do so with sippy cups??It is beyond me... I guess if there's one thing you're going to lose, sippy cups are preferable to most things.