Design: How do you decide when items are related vs when they are attributes? I've been trying to figure this out for a while.  Let's say you have a person, for example.  An attribute of a person is his or her social security number, right?  But a person also has a social security number.  So, in an ER diagram, you could draw a square box for person and a square box for ssid, and you could connect them by a diamond, has.  Alternatively, you could draw a circle, ssid, and you could connect it to a square person box.

This isn't just true of ssid, though.  It's possible to draw either-or with concrete things, like a car, a pet, or a phone number, or with conceptual things, like a phone number, a friendship, or a mood.  So, where do I draw the lines?  Is the object of design just to get everything down on paper and looking nice, or should I be using a guide?
One rule of thumb is that if it is 'physical' (concrete as opposed to abstract) then it is an entity otherwise it is an attribute.

Another rule of thumb in ER modelling is that an table EITHER models an entity OR a relationship between entities but never both. There can be other kinds of table, of course e.g. it is sometimes conducive to put all attributes of the same type (e.g. colors) in a table, known as a lookup table (i.e. is neither an entity table nor a relationship table but is still valid and useful).