where did these come from?

The other day I was thinking about some phrases that we often use that sometimes don't make sense. Anyone know where these phrases came from?

--He/she was as happy as a "clam" (since when are clams happy?)

--Keeping up with the Jones'

--You need to share (ok, when you've got one thing, how the heck are you supposed to share it?! When one kid has it, that's it...two people can't play with the same thing at the same time!

--Going to school, having a job, and participating in extra-curricular activities will help you become a more "well-rounded" person (what is well-rounded anyway?)