What a great thread, alexis! I like your original example - never heard that before!

I like "dumber'n a barrel of hair" - which apparently gets used in Texas. The Globe and Mail, "Canada's National Newspaper!", occasionally runs an item called "word watch" on the back page of the front section - sometimes it's very amusing.....that's where I got this expression from.

I haven't heard "sharp as a whistle" but I like it. I HAVE heard "sharp as a tack," "sharp as a knife," and "so sharp he'll cut himself." Conversely, people sometimes talk about someone as being "not the sharpest knife in the drawer." But whether that means that person is dumber'n a barrel of hair, I guess I'll never know.