A flag should have a ``certain air of mystery about it,'' says Smith. ``If you have a symbol that is too explicit, everybody looks at it and says `Oh, yeah - there's the tree, and there's the grist mill.' There's nothing left to the imagination. But a good flag design allows everyone to see his views in it.'' For examples of bad design, says Smith, look at almost any city or state flag. Flags can be too simple, though: Poland's red and white stripes, for example. Other flags look too much alike: The banners of Luxembourg and the Netherlands seem identical. Only a shade of blue - and their proportions - differentiate them.