Words is funny, ain't they?

I've spent most of my life studying language- both my own and foreign- and still find myself perplexed from time to time. What's come to mind this afternoon is: How do we get silly phrases that don't make much sense? Why do we say them without really thinking about what they mean?

I guess it all started when I e-mailed a coworker that I'd "keep my eyes peeled." As I looked at the words I thought about how odd that phrase is. Not only is the idea of peeling your eyes pretty disturbing, it's also counterproductive to alert vision. So why do we say it?

This occurs in all languages, and I'm fascinated with why. Colloquial speech and slang permeate a society without much notice, in the same way fashion slowly leaks in. I'm sure one day someone woke up in 1987 and said, "Oh dear God! What am I wearing?!" Similarly, we say things like "he hit on me" without thinking about the literal meaning. He hit on you? I certainly hope not, and if so you should not have given him your number.

Does anyone have thoughts or explanations? Maybe it's not that complex, but at the end of a Monday the mind wanders to places of inquiry- at least mine does.