My (limited) understanding of fish behaviour is, like babies, they will try anything in their mouth to see if it is edible or not. Therefore anything could be a potential bait. I think Rod Hutchinson used black jelliebabies to mimic frog spawn. My first fish on the Royalty was taken on a bare bright gold hook trotting to ensure I had the correct depth to avoid bottom weed. Catfish have been known to take footballs and one angler reported catching a carp on a floating rhododenren leaf when it was taking other surface leaves and rejecting bread and floaters. Another angler caught a pike on his pike bung float by fitting a treble to it when a pike kept attacking the float and ignoring the livebait below. I have even seen carp taking cigarette butts and blossom and caught carp on potato! I would not fish with leaves or cigarettes because obviously some baits are better than others - but I might just try a marble to see what happens on a prolific barbel stretch! Some of todays normal baits were yesteryears unusual (or unknown) baits and many anglers have forgotten old baits such as bread, cheese and minnows. I think factors for the fish include curiosity, availability, taste and nutritional value. On one stretch the best bait is chips from the local Chippy because so many people throw a chip or two over the nearby bridge to feed the fish! If you can eat it so can they and sometimes changing the hookbait to something more unusual can bring an instant response when other baits have blown.