We've studied them over burgers and portobello sandwiches; we've dipped them in mayonnaise, mustard, and ketchup; and we still can't figure out what Rudy's does to its fries to make them so good. The exteriors are crispy, glossy, and golden, almost as if the spuds were battered, and they don't even need ketchup or mayonnaise. But you have to be a Mickey D's skinny-fry kind of guy to appreciate Rudy's fries, not a thick-as-your-arm, potato-wedge-fry kind of guy (for those, go to JoJo on Piedmont Ave.). Rudy's, the anti-retro Emeryville diner with the most punky-attractive waitstaff in the East Bay -- if you drool over that tattooed, plugged-ear look, which we do -- will substitute mixed greens or potato salad for deep-fried starch. But why would anyone do that?