Final answer:
Iron nails are attracted to magnets because iron is a ferromagnetic material and becomes locally magnetized near a magnet, but unmagnetized iron nails don't attract each other as they lack aligned magnetic poles.
Step-by-step explanation:
Iron nails are attracted to magnets because iron is a ferromagnetic material, which means it can form permanent magnets when exposed to a magnetic field. However, iron nails are not attracted to each other because they are not magnetized; they do not have their own north and south poles aligned in a way that would allow them to attract each other like a magnet does.
When a magnet is brought near an unmagnetized ferromagnetic material like iron, local magnetization occurs with unlike poles closest, thus creating an attractive force. On the other hand, unless iron nails are deliberately magnetized by stroking them with a magnet, for example, they will remain unmagnetized and will not exhibit the attraction you observe between magnets.