The game was designed that way, and is the same in the Windows, Mac, Palm and Pocket PC versions, as well. The first slot is for food, the second for medicine, the third for chemicals, and the fourth for eggs. When you select something to purchase that would replace an item already in the toolbox, the game asks you if you want to replace the item occupying that slot. Unfortunately, many players automatically click OK without seeing that it's asking something other than if they want to purchase it.